Chambering and Installing an AR-15 Barrel

The following documents the build of an AR-15 upper chambered for the .300 Whisper, also called the .30-221 Remington or .300-221.  This build utilizes a factory-made upper receiver frame (the same one used for .223), so the predominate focus is on chambering / fitting a barrel blank and setting up the gas system.  A knowledge of the working of an AR-15 is assumed.  If you don’t understand the AR-15, study up before attempting to follow along.  I also assume you have machine tools and know how to use them – this is not a machining tutorial.

Step 1: Assemble the materials.  It’s best if you have all parts on hand before starting.  You’ll need:

  • A complete AR-15 upper receiver including the dustcover, forward assist (if your receiver uses one), charging handle, bolt and bolt carrier.  All of these parts used are unmodified from their .223 versions, so little detail is given to them.
  • A .30 caliber barrel blank.  I started with a Shilen Match .30 ChromeMoly barrel blank, 28″ long and 1.26″ in diameter.
  • An AR-15 barrel extension.  It’s possible to machine one of your own, but they’re presently under $20 from Brownells…  This piece provides the locking lugs, feed ramps, and hole for the indexing pin.
  • A low-profile gas block.  We’ll use a short (‘pistol length’) gas tube, so the gas block will need to fit underneath the handguard.  A standard gas block may be modified, or a low-profile one may be used.  An adjustable model is probably best if you’re unsure of gas settings for the caliber you’re building.  I’ll be modifying a JP adjustable block to fit.
  • A gas tube.  Almost any gas tube will work, you can cut it to the needed length. (Cut it at the end that goes into the gas block, NOT the end that mates with the bolt carrier key.)  An adjustable gas tube is desirable if the gas block is not adjustable.
  • A hand guard.  The ‘float tube’ will be the easiest to work with and is what will be used in this description.  I ended up using a different handguard than the one shown in this picture.
  • (optional) Whatever muzzle attachments you may want (flash hider, compensator, or suppressor).  You’ll want to at least know what thread is needed.
  • Chamber reamer.  You can buy a reamer from any of the major suppliers: JGS , Pacific Tool (where I got mine) or Clymer.  You can rent a reamer from 4D.
  • Go/No-Go gauges.  Available from the same suppliers as your reamer.  A ‘combat fit’ can be achieved using properly sized brass, but gauges are desirable and they’re available for this caliber, so that’s the proper way to go.
  • Other chambering tools.  This includes a means to measure (precisely) the movement of the tailstock ram, as well as a spider, or steady rest, if required to chamber your barrel.

 

Parts and tools for building an AR-15 barreled upper receiver

Step 2: Develop a plan. Before starting any metalwork it’s good to know what the end result should be.  In particular, figure out where you want your gas-block and/or gas tube to be installed and make sure your barrel contour is planned to fit it at that point. (We’ll contour the barrel later, after it’s chambered.) Also, figure out how to reach any gas adjustment ‘knobs’ when the whole thing is assembled (hole in the float tube?).  Determine the barrel length that you want and what contour it should have.  Draw it up!  Don’t rely on your memory.  A napkin sketch with dimensions is all that is needed.

I want a relatively short barrel, and the minimum legal length is 16″ (as with all legal advice in this document, verify it yourself for all the laws pertaining to your locale before proceeding).  For this reason I will aim for 16.5″ (the extra half inch is buffer in case I ever have to deal with lawyers.).  I intend to use a minimum length gas tube (“pistol length”) and I want threads for a future silencer which will cover the last 4″ of the barrel (The silencer will cover the last 4″, the threads will only be about 3/4″).  My starting gas block has a 0.936″ bore, so I need that diameter where I attach the gas system.  I intend, eventually, to drill a hole in the float tube to access any gas adjustments.  Finally I intend to make a custom gas block with three settings: Subsonic loads, supersonic loads, and ‘off’ for manual cycling of the action.  The following drawing shows my plan.

The plan for this barrel's pertinent dimensions

Step 3: Prepare to chamber the barrel. The blank I bought (from MidwayUSA.com) is 28″ long!  Needless to say, some I don’t need all that length, so I’ll cut it shorter.

 

The barrel blank cut down to working length

Step 4: Buy or turn an indicating rod.  You’ll want to center on the bore for the next few steps, and an indicating rod is just the way to do it.  If you have a pin gauge set, you can use the appropriate pin, otherwise carefully turn a shaft to the diameter of the bore.  Make your finishing cuts with a very sharp tool that has lots of rake, well centered on your toolpost.  You want a snug fit that with no discernable wobble when inserted.  If the finish isn’t excellent, you’ve failed.  Some brief trial and error resulted in a 0.300″ pin for my barrel.

Step 5: Thread for the barrel extension.  The next several steps (including chambering) should all be done with the same lathe setup.  There are probably as many setups for chambering (and it’s precursor steps) as there are barrel makers, but I’m going to show you an easy setup that works well for the sort of barrel I’m doing.  There are many books that suggest either using a steady rest, or a ‘spider’ at the back of the headstock to keep the barrel concentric.  We’ll dispense with that stuff and chamber directly in the 4-jaw chuck with no muzzle-end support.  My rationale is that the barrel is short and, at 1.26″ diameter, very rigid, and we’ll be running at relatively low speeds.  Note that I use a big lathe and chuck and the jaws of my chuck have 2.5″ of bearing surface on them.  If you have a much smaller machine it may behoove you to provide some muzzle end support.  I leave that to your discretion.  Some purists will scoff, but I believe this is more accurate than most steady rests.  That said, clamp the barrel in the 4-jaw, with ~1.5″ protruding from the jaws at the breach end, insert your indicating rod, or gauge pin and indicate it true to less than 0.001″.  Do NOT indicate on the outside of the barrel – bores are seldom perfectly concentric with the outside diameter.  Either use an indicating rod, or indicate directly on the grooves (not the lands).  If indicating on the grooves, align your indicator to be on the same axis as the jaws you’re adjusting.  If your chuck can’t get to within 0.001″, and you are practiced at manual centering, it is likely that your chuck jaws aren’t square with the bed.  Correcting that difficulty is beyond the scope of this work, but fix it before continuing.  If you’re not practiced at manual centering, try and try again – centering to within a mil isn’t really that hard:  Number your jaws 1-4 such that 1 is adjacent to 2 etc, if they’re not already numbered.  When starting, focus only on jaws 1 & 3.  Adjust until the difference between those jaws on a dial indicator is just a few thosandths.  Now repeat for Jaws 2 & 4.  Go back to 1 & 3 and ‘fine tune’, continue until satisfactory aligment is achieved.  While it sounds tedious, this process is relatively quick once you’ve done it a few times and you should have a readout of less than 0.001″ TIR (Total Indicated Runout) quickly.  An indicator holder like the one in the picture (homemade) is super handy for this.

Indicating the bore to center of rotation

Now face off at least 1/4″ (0.250″) of the barrel.  Depending on how the barrel was made, the manufacturer may have left some relief in the beginning of the barrel, probably a countersink as well.  Get rid of all that.  clean the ‘throat’ of the bore so no burrs remain (sandpaper on a stick is good here), reinsert your indicating rod and verify your alignment.  Adjust if necessary.

The standard AR-15 barrel extension is threaded 0.810″-16.  Turn a tenon to 0.810 diameter for a length of 0.620″.  It’s easiest to cut the tenon a little long, then face to final length.  Cut a 0.100″ thread relief where the thread will end.  Try to make the thread relief only as deep as the thread will be – remember you’re removing metal from around the chamber. A 16 TPI thread is theoretically 0.054″ deep.  While you’re here, make a clean-up pass over the full exposed length of the barrel.  Aim for a nice finish.  This will provide a section that is concentric to the chamber that can be used for setup when countouring the barrel.  You can get an idea here of how much metal we need to remove when we contour…

The thread tenon is cut and ready to be threaded for the barrel extension

Thread the tenon 16TPI, using your barrel extension as a gauge.  I aim for a snug hand fit – that is, I should be able to turn the barrel extension on, all the way, by hand, but otherwise I’d like it tight.  Derrick Martin from Accuracy Speaks says to cut until it screws on easily, so if you get it less than snug, don’t worry.  (If it’s sloppy, cut off the tenon and try again – that’s why we left some extra length allowance in the barrel.)  Finish by turning off the first thread.  This keeps any burrs from the thread end from interfering with the bolt.

The barrel tenon thread completed

Step 6: Chambering (part 1 of 2).  Alright, this is the ‘big’ step.  It’s really pretty simple, so don’t get psyched out about it, just go slow and you should get good results.  There are lots of ways (and opinions about how) to chamber a barrel.  The key thing is to guide the chamber reamer in along the centerline of the bore, and control the depth precisely.  Anyone chambering a barrel should have a copy of John Hinnant’s book The Complete Illustrated Guide to Precision Rifle Barrel Fitting. Also The The Complete Guide to AR-15 Accuracy by Martin and Tillman is an excellent AR-specific addition. (Links to both of these books at the end of the article). Hinnant’s book is the most complete description of chambering that I’ve come across. I won’t be following his advice directly, but it’s still good advice.  Some ‘smiths advocate achieving a perfect tailstock alignment with the center of rotation and holding the reamer in a fixed position.  Others (myself included, for this build) will use a ‘floating’ reamer holder that lets the the pilot on the tip of the reamer find the center and allow it to do the guide work.  There are a lot of concepts about how floating reamer holders can be made – I’ll be using the one I detailed here.  This holder allows the reamer to adjust angle and offset from the tailstock, but maintains a consistent depth reference.  When stopping the cut of a reamer, forward pressure should be relieved without backing the reamer off until the lathe has come to a complete stop.  I think this is good advice when using any reamer holder.  Backing off allows gaps between the chamber wall and the reamer which can potentially allow a chip to ‘roll over’ the reamer edge, dulling or damaging the reamer and possibly scoring the chamber wall.

Reamer notes: There are several different types of reamer: roughing, finishing, and sizing.  There are also different materials and pilots.  In the material category, the more you pay, the more you get.  Unless you’re going into production (or have an oil-pressure system) I suggest getting the ‘cheap’ standard reamer material, which is High Speed Steel (HSS).  There are also fixed and replaceable pilots.  If you use the aligned tailstock – fixed reamer technique the fixed pilot is probably OK.  If you use the floating reamer holder, I recommend the rotating pilots.  I think most of the manufacturers only do oil cuts in pilots for their carbide reamers, so I’ll drop that discussion here.  Of the types of reamer available the only one that is a must is the finishing reamer.  You need a finishing reamer to cut a chamber to its final size.  You don’t need a roughing reamer – the rougher is a nicety, and will help extend a finishing reamers life if you’re cutting many chambers, but you can cut the whole chamber with a finisher if you like.  The sizing reamer is for making sizing dies.  Nothing else.  It’s an undersize reamer and if you want to make your own sizing dies that’s the way to do it, but it doesn’t have anything to do with chambering.  Because I can probably count on one hand the number of chambers I’ll cut, I’ll use the standard finishing reamer, with a replaceable pilot, and I’ll cut the whole chamber that way.  If you’ve watched the AGI barrelling videos you might have seen Darrell Holland rough out a chamber with a drill bit.  This is a sleazy roughing reamer and could reduce final accuracy.  If you do this, the hole cut by the drill quickly becomes the pilot hole, as it has more bearing surface than the reamer pilot, and if it wobbled at all, you may get a less accurate chamber.  (Truth be told there are precious few people who can shoot up to their rifles potential, so this isn’t a big issue, but I prefer to give my rifles as much potential as possible for this operation.)  Additionally, if you’re not cutting a lot of chambers, the wear you save on your finishing reamer will be inconsequential.  We now return to your regularly scheduled program…

Cutting the chamber is as easy as slowly feeding the chamber reamer into the blank.  Using the type of reamer holder I have, if anything grabs, I just let go of the handle and let the reamer and holder spin.  (Same if you have the reamer on your tailstock center and a wrench on the reamer.)  Set up any apparatus you may need to measure the feed on your tailstock.  Chambering can be done with just the tailstock dial, but most people prefer some sort of indicator attachment.  Mine is pretty typical.

The arrangement I use to accurately indicate tailstock extension

I like to start the chamber (get much of the cutting done) before zeroing the tailstock indicator.  Running the reamer in is a slow process, if done correctly.  Here’s the touchy part (don’t get nervous it’s easy).  You want to feed the reamer in slowly, but fast enough that it’s always cutting.  Set your lathe for a slow speed – something in the 100rpm range is great if your lathe can do it.  Lube your reamer with cutting fluid before starting.  Feed for no more than 1/16″ (0.062″) then stop, holding the reamer in place until the lathe has finished turning.  Withdraw the reamer and  thoroughly clean it.  Clean the reamer and the chamber each time – clean them completely.  Use solvent if necessary. Use compressed air if you have it.  If you don’t have a compressor a can of compressed air (available from office supply shops for dusting computers) will probably be worth the price.  The 1/16″ infeed is just a guideline.  Definitely stop if and when you see chips filling the flute.  The goal is to never let the chips become packed in a flute.

The reamer pilot inserted and ready to being reaming

Continue this process until the chamber is close to your roughing depth. You can determine roughing depth by inserting a sized case – but don’t go until it’s flush – it should stick out about 0.175″ or even a little more.  I coated my case with DyKem and scribed a line at 0.200″ to use as a test.  When the case can be inserted to the line, the roughing is done.  Zero the tailstock indicator in this position.  From here on out, the tailstock must stay in place, or you must be able to return it to a precise location or the depth reading won’t mean a thing.

Reaming the chamber

Here’s the test case – the chamber now swallows it up to 0.200″ from the base – time to check headspace!

The rough depth has been reached, as indicated by the marked case prepared for gauging

Step 7: Chambering (part 2 of 2).  Finish chambering – getting your headspace right.  There are several ways to get your headspace, this is just one.  I assume the use of gauges.  Strip your bolt of extractor and ejector. Insert the ‘Go’ gauge in the chamber, after cleaning it thoroughly. With the bolt held in the barrel extension, screw the barrel extension on to your barrel until it bottoms out on the Go gauge with just light pressure.  Using shims, check the distance remaining between the barrel extension and the tenon shoulder.  This is the amount that you still need to push the chamber reamer in.  I’ll repeat this measurement again (possibly several times) before finishing, but this is the only time I’ll describe it.

Checking the chamber depth and measuring remaining cut

Proceed in smaller steps until you just barely achieved a ‘Go’ gauge fit.  Check the ‘No Go’ gauge.  With the barrel extension fully installed, you shouldn’t be able to insert the bolt while the ‘No Go’ gauge is installed.  If you can – well that’s why we left that extra barrel length.  If you’re just barely off, you can cut the shoulder back 0.005″ and try again, but you can’t do this if you’re way off – you have a thread relief that will encroach on the area where you want thread support.  If the no-go gage just barely works, you’ll probably take up the slack with the crush when the extension is tightened on.  Real pros don’t need the no-go gage – they just cut until the go gage fits.  This is the ideal situation, but the no-go gage is about $27, and cheap insurance in my opinion.  I definitely recommend the no-go gage if you’re inexperienced.

Completed chamber

Step 8: Cut the chamber relief. Now cut an chamber relief (or ramp) at an approximately 60° angle (that’s the ‘included angle’ your compound will be set to 30°).  Open the chamber to 0.540″.  This will help make feeding reliable.

Step 9: Turn a reference journal.  You should still have an inch or so of full diameter barrel protruding from the lathe.  Turn this to the diameter you want at the end near the chamber. (1″ for me). This will provide a reference surface to chuck to when contouring.

Starting the contour provides a reference journal for later setups

Here’s the barrel as it is now.  You can see that we have a lot of metal to remove when we contour (that piece in front of the threaded section is the *thickest* part of the finished barrel!)

The barrel blank after the first setup, along with two great references

Step 10: Cut the barrel close to final length. Eventually I’ll face the barrel to it’s final length, but since I’ve left so much extra it’s time to saw off a bunch of the surplus.  I’ll cut my barrel to 17″.

Step 11: Countersink the muzzle.  This isn’t really the muzzle, but the end that will be the muzzle.  Put it in the 4-jaw chuck, center then face it off.  Use sandpaper on a stick to remove burrs, insert the indicator rod and center precisely.  Now, put an indicator in your compound and adjust the indicator until it shows no deflection when running down hte taper of your tailstock center.  (Use the compound for advancing the feed.)  Put a sharp boring cutter in the toolholder, and using the compound to feed, cut a liberal cone for your center to rest in.

Aligning the compound precisely to the tailstock center

I used a boring bar for my center cut, feeding in with the compound until a sufficient area was made for the tailstock center to rest in.

Cutting a chamfer for the center using a boring bar

Step 12: Contour the barrel.  I’ve saved contouring for now so that we had some wiggle room while chambering, but we now have a chambered barrel, so it’s time to get careful, even though we’re in the easy steps.  Contouring is dull – there’s a lot of material to remove in my case, and the cuts aren’t challenging, but that’s when the time is ripe to make mistakes, so think about it.  I’m switching over to my 3-jaw chuck, but if yours isn’t accurate, use a four jaw, but indicate on part of the journal we cut in step 8.  My 3-jaw chuck is sufficiently accurate (< 0.003″ TIR) so I’ll just clamp the the journal.

Here’s the setup for contouring.  That’s the muzzle end in the tailstock.

The barrel set up for contouring

After waiting on several interminably long cuts, the barrel is contoured to the major diameter (1.00″).  If you notice that flange near the chuck – no I don’t have the barrel extension installed – that’s just a little bit that I can’t reach with my cutter in its current position, so I’ll clean it up later.  (Shows how much material has been removed though!)  I’m altering my previously posted contouring plans because I went with a 7″ gas tube so I”ll only make the 1″ diameter section 3.6″ long measured from the extension.

The barrel is now 1.00″ for the entire length.

 

Full diameter contour complete

We’ll want to take several ‘spring cuts’ (cuts without advancing the cutter) as we get close to our final diameters because the barrel will flex a little.  My  largest diameter is 1″, so I’ll turn the whole blank to 1″ diameter, sneaking up on the last cut to get it just right.  My gas block fits a 0.935″ diameter, so I’ll sneak up on that for the section of the barrel that is that size or smaller.  In my contour design I left enough room that I can cut to that measured area by eye, and don’t have to get overly concerned about thousandths in terms of length of cut.  Here’s the contoured barrel.

Barrel contour complete

My next step is where I’ll put threads for a suppressor.  That diameter is 7/8″.  I want the threads to be 3/4″ long, so I turn my final diameter of 0.750″ for the remaining length.  That’s it! The barrel is contoured!  To prepare for threading I’ll cut a thread relief 0.100″ long and 0.100″ deep at the back end of the thread tenon left by contouring.  (That should still be a diameter of 0.775″ – larger than the barrel end diameter of 0.750″.

Thread relief for the suppressor threads

Step 13 Thread the barrel for muzzle attachments. This is optional, and if there are no threads on or near your muzzle you can obviously skip this step.  I’m going to thread the section previously described 7/8″-20 for a suppressor.  Later, I’ll make a stylish nut to cover the threads when a suppressor isn’t in use.  First, I’ll use the steady rest for extra support.  Here it is going on…

Setting up the steady rest in preparation for cutting suppressor threads

Here’s the steady rest set up to support the thread cut.  Note that the tailstock is still present.

The steady rest in place

And here’s the threaded tenon for future attachment of a silencer.

The threaded section for attaching a silencer

Step 14: Cutting to length and crowning.  The hard work is done!  Measure the length of the barrel.  I do this without breaking the previous setup.  Install the steady rest (backing off the tailstock) and get the muzzle end running true.  Face the muzzle off until you reach your desired length (16.5″ in my case).

The muzzle is faced to length in preparation for crowning

Now, sharpen and hone a fine tool to a razor edge – you can’t be too sharp for this step.  Cut the crown.  I’ll leave the details to your imagination – there are quite a few crown styles, but in my opinion all that matters is that the cut is clean.  I also like it recessed somewhat so that scuffing the muzzle on a prop or the ground won’t damage the crown.  My recess is very shallow and small, this is part of my plan to mate to a suppressor.  Turn the outside corner to a slight chamfer. (Later, if you think the crown needs improvement you can polish it with some valve lapping compound on a rounded piece of brass in a drill.)

The crowned barrel

Ta-da!  The barrel is basically done!

The completed barrel next to a piece of the original stock

 

Make a thread protector. To start the thread protector recall that my threads are 7/8″-20.  So I want something large enough to cover that.  I’ll start with a piece of 1.125″ roundstock(in this case the piece of the barrel that was cut off when trimming to length – that way any finish will match my barrel).  In order to index it, I’ll clamp it in a 5C Hex collet block.

Stock set in a collet block for easy indexing

Using my largest ball-end mill I’ll machine grooves down the side.  My grooves are 1/16″ deep.  The length doesn’t matter as long as it’s in excess of the 3/4″ finished length I’m after. (Leave some extra for facing, etc.) Here’s the first groove cut:

Cutting grasping grooves in the decorative nut

Here’s the profile finished.

Grasping grooves completed

Cut the piece to a length a bit longer than the nut.  Because I’ll want to check the fit as I’m threading, I’m roughing the piece to length now.  If I didn’t have to stick my barrel 4″ in to try the threads I’d part it off after the other operations were finished.

Chuck the piece up in the lathe and face the end – I chucked across the flats, not in the grooves. Add a chamfer for cosmetics.  I’m using High Speed Steel (HSS) cutters for all this work because carbide hates an interrupted cut.

The faced and chamfered nut in progress

Bore a hole to the minor diameter of your threads.  The minor diameter for my 7/8″-20 threads is 0.821″

Hole bored in the nut in preparation for threading

Now thread the hole 20 TPI.  I like to mark my cross slide handle with the direction to turn in order to back off.  I do this because I cut so many external threads that it’s easy to revert to ‘instinct’ and crank the wrong way when cutting internal threads.

I mark the cross-slide to remind me that I need to crank 'in' when cutting internal threads

Thread the hole, trying out the barrel for fit as you get close.  (Make sure you clean the threads well before testing for fit.)

The threaded nut

Reverse the piece in the chuck, face it off and measure the resulting length.  Calculate how much more you need to remove and face it to final length (0.75″ for me).  Chamfer the outside edge to match the other chamfer and it’s basically done!

Facing the nut to length

And here’s how it looks on the barrel.

The completed nut installed on the barrel

Attach the barrel extension. I start by attaching the wrench I made to the barel extension.  It’s a good fit, so I throw some rosin on it (both halves) and attach it to the extension.  If you can see any brown in the picture, that’s the rosin.  Tighten the screws tight.

Installing the barrel wrench on the barrel extension

I intended to use my barrel vise to hold the barrel during this operation, but wouldn’t you know it – I have seven bushings for it and none of them are 1″.  Go figure.  Anyhoo, I decided to improvise with a couple of pieces of 2×4 wood.  I screwed them together temporarily and drilled a 1″ hole with a hole saw.  (This required separating them once to chisel out the part that was being cut.) Here’s the hasty barrel vise jaws.

Improvised barrel vise jaws

I coat the inside with a liberal amount of rosin and clamp the two halves together in my big vise.

Barrel clamped in the improvised vise and ready to tighten extension

Before I torque everything on I’m going to make some witness marks on my barrel, wrench and wood so I can detect if anything is slipping when I snug it up.

Witness marks to indicate if the barrel is slipping in the vise

Now attach the torque wrench to the barrel extension wrench and torque to 140 ft-pounds if you can get there.

Torquing the barrel extension to 140 ft lbs.

 

 

Verify your headspace again, using the bolt and ‘go’ gage.  My headspace didn’t pass, so I had to take the reamer (by hand) and give it another quarter turn in the chamber to get it to go.  This is no big deal and still produces a quality chamber.

Drill the gas port. The first part is to verify that the vise jaws are parallel to the ‘x’ movement of the mill table.  Then clamp the barrel so it’s level and the hole in the barrel extension is at top dead center.  I established top dead center by centering the barrel under the quill and chucking a 0.120″ gauge pin in the drill chuck then pecking at the barrel until the pin would go into the hole.

Finding Top Dead Center (TDC) using a gauge pin to locate the hole in the barrel extension

Leaving the Y-axis in line with the alignment pin hole, move down the barrel to the location of the gas port, and drill it to size.  No matter what caliber you’re doing, there’s no point in drilling the hole larger than 0.125″ as the gas tube is only 0.120″ internally.   Don’t worry about any internal burrs – they’re removed automatically in the first few shots.

Drilling the gas port

That’s it for this setup.  Remove the barrel and tap the barrel extension alignment pin into place. Trim the end of the pin a little below flush with the flange on the barrel extension if necessary.

Barrel extension pin tapped into place. This one is a bit short, but it's still long enough to do the job

Modifying the JP adjustable gas block. The JP is a little too big to fit under my current handguard (and since I’m using an extra-short gas tube it has to fit under the handguard).  I’ll prune the top off it before continuing.  Here’s the unmodified block:

An unmodified adjustable gas block from JP Rifles

Fit the JP gas block.  The gas block should already fit over the barrel and the gas port should line up, but the JP gas block is too tall to go under the handguard as previously mentioned.  I just mowed the top (the accessory rail) off downto almost flush with the with the slots in the rail (about 0.100″) and then machined the sides to match the width of the body.

Milling the gas block to a lower profile

Chamfer the edges.  I chamfered the edges by about 0.030″ each side.  This may or may not be necessary to fit your handguard.  It wasn’t strictly necessary for mine, but a little more clearance seemed like a good idea.  Before wrapping up this step, test the fit under your handguard.

Chamfering the sides of the gas block for clearance under the handguards

Install the gas tube in the block.  This is just following the directions that come with the JP block, but I’ll detail the important steps in case anyone is making an adjustable block of their own.  Here’s a pic of the gas-block end of the gas tube showing the gas port and the pin hole.

The end of the gas tube that mates with the gas block

Push the end of the gas tube into the gas block until the cross pin hole and gas port hole are aligned.  Drive in the roll pin that holds the pieces together.  There’s a 6-32 threaded hole for the adjustment screw.  Run a 6-32 tap down this hole to put thread impressions into the hole in the gas tube, so the adjustment screw mates nicely.  Replace the adjustment screw and the tube is installed.

Tapping the gas block and tube for the adjustment screw

Install the barrel.  We’re finally ready to finish assembly!  Insert the barrel into the receiver, with the alignment pin going into the slot in the top of the receiver threads. Smear the receiver threads with a good moly grease or anti-seize compound. Screw the barrel nut on by hand until it is snug.  Now clamp the receiver in a receiver vise and using a spanner (like the one made for AR work) tighten the barrel nut to 31-35 ft-lbs.

Securing the barrel nut

Finish barrel install. Finish by tightening until the gas tube hole is clear of obstruction. A pin the diameter of the gas tube, or a little larger is handy for this.  You want a free fit of the pin into the receiver.

Using a pin to verify alignment of the clearance slots in the barrel nut and the gas tube hole in the upper receiver

Install the gas block. Push the gas block on and insert the gas tube through the hole that aligns with the top of the receiver.  Now is a good time to insert your bolt carrier group and make sure the gas tube enters the receiver straight and that the carrier key doesn’t rub or bind on it.  Assuming your barrel and gas block are both flat an easy way to align them is to lay them on a flat surface and align the tops of the receiver with top of the gas block.  A 5/32″ center punch or sharpened rod can be used to mark where your clamping screws go.  I intend to replace this gas block eventually, so I’ll only drill for one set screw, the other two will just bear on the barrel.

Marking the location of set screws for securing the gas block in place

Secure the tube.  Finally, if you’ve had a float tube before you know they can come loose at the most inopportune times – like in an IPSC 3-gun match when you should be making the shot and instead you end up thinking “my handguard is coming loose”.  To prevent that I’ll put a small setscrew in the handguard to secure it to the barrel nut.  Screw the handguard on hand-tight and (I like to put the setscrew in the bottom so I don’t have to worry about interference with the gas tube) drill a pilot hole through the handguard and tap it – I’ll use #4-40 tap.  Replace the handguard, and put the set screw in with enough force to mash the threads underneath it.  I’m not going to show this step, because I don’t intend to do it for a while, but that’s the plan.

This project doesn’t require any work to the bolt or other parts, so that wraps it up. Make sure you clean the barrel well before test firing!

The finished upper

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3 Responses to Chambering and Installing an AR-15 Barrel

  1. Pingback: Building an AR-15 in .458 SOCOM | Firearms Designer

  2. Scott says:

    Are you going to detail out the can used on this barrel?

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