Designing a Gas-Operated Pistol Caliber AR-15.

Let’s design a gas-operated AR-15 in .45 ACP.  Obviously that means starting with an AR-15, so much of the design is done already. Because the system will be gas operated, and therefore have positive locking, it should be able to accommodate extra hot loads, even the .45 Super.

There are still some design challenges, however.  The change in caliber, and adaptation of a rifle recoil system to a pistol round, results in some design considerations:

  1. Where should the gas port be placed and what diameter should it be?
  2. What is needed to accommodate a .45 on an AR bolt?
  3. What type of magazine will work?
  4. What changes are necessary to the feed ramp / barrel extension to work with a .45?
  5. Where should the magazine be placed for optimal feeding?
  6. What additional considerations are there?

This is a work in progress, and I’m attempting to expose my thinking as I proceed, so the following ideas may be modified before we’re done, but they are my starting point.  Any of them are subject to change as the design progresses.

Having identified as many design considerations as possible, the next step is to address them, one by one.

Where should the gas port be placed and what diameter should it be? The first half of the question is the most important as the second half is a bit fudge-able.  The gas port must be far enough to the rear to get *enough* gas to cycle the bolt, but too much gas will result in increased recoil and perhaps damage to the bolt-cam pin. I intend to use internal ballistics simulations software (Quickload) to determine the ideal position for the gas port.  The size of the gas port is another matter.  The inner diameter of an AR gas tube is 0.120″, so there’s no point in going much larger (a little larger might compensate for some mis-alignment of the gas block).  Three approaches readily come to mind: 1) Drill a small hole, and increase its size (up to 0.125″) until the rifle functions, 2) Drill a 0.125″ gas port and use an adjustable gas block, or 3) Use an adjustable gas block and once finding the correct setting, correlate that to a specific port size.  A fourth method might be to make calculations to determine the ideal size, but we’d likely end up fine-tuning that calculation using one of the aforementioned methods anyway.

What is needed to accommodate a .45 on an AR bolt? Fortunately, I don’t have to re-invent the wheel here.  This has been done a number of times, and a 7.62 x 39 bolt can be opened up to accommodate a .45 case head while still leaving a web between the bolt lugs.  The web is a safety consideration, as it helps seal the chamber in the event of a ruptured case

Two bolts. The one on the bottom has had the face machined open too far and as a result there is no longer a web between the lugs

What type of magazine will work? Obviously the magazine to be selected must hold .45 ACP cartridges.  It must also allow enough clearance at the rear to permit an AR bolt to pass enough between the feed lips to pick up a round.  It must be made to work in an AR-15 receiver.  Common .45 magazines, and some of the considerations for their use are:

  • 1911 magazines – low capacity, single-stack means simplified feeding, angle might be annoying.
  • Grease Gun magazines – Cheap.  Very wide relative to the AR receiver.  Double-stack may complicate feeding.
  • Thompson “Tommy Gun” magazines.  Mid price.  Wide, but not as much as the Grease Gun mags.  Double stack may complicate feeding.
  • Uzi (.45) magazines – Very expensive and moderately hard to find.
  • 9mm Sten magazines – Cheap.  Readily available.  Single-stack at the feed mouth.
  • Custom magazines.

I considered the 1911 magazine as a last resort.  There are many high-quality 1911 magazines on the market, but the quality peters out when capacity exceeds 10 rounds, and I really wanted higher capacity, in keeping with most AR carbines.
The Grease gun magazine is ubiquitous and inexpensive, but about as wide as an AR receiver.  At this point, the GG mag is my #2 choice.

Tommy gun magazines are common and relatively inexpensive. They are my first choice after reviewing what’s available. I saw new ones, still in the packaging, today for $30 ea.  Not a great deal, but not prohibitive, and they appear to be higher quality in general than the Grease Gun magazines.  Unfortunately they are still pretty wide compared to an AR receiver.  I think you could machine a standard AR lower receiver forging to accommodate them (0.05″ thick walls, but reinforced by ribs at top and bottom) but it’s not the greatest solution.  I’ll probably go with a completely fabbed mag-well section.

Uzi .45ACP magazines are $150 or more each, and not all that common.  ‘Nuff said.

9mm Sten magazines have been reported to work with .45, but the round sits lower in the magazine, meaning some additional magazine work is likely necessary.  They’ll also have reduced capacity, though at this point I haven’t bothered to figure out what that is.

Custom magazines would be very expensive (it takes a lot of tooling to make a magazine).  The work would not be worth it unless I was planning for commercial production, and magazine design and manufacture is fraught with problems.  That’s why there are so many junk magazines on the market.  Whenever possible I will pick an existing magazine to use in my designs.

So I’ve tentatively selected the Thompson SMG magazine.  By doing so, I have opted to machine a custom lower reciever that will work for the wider magazine.  I could machine a forging to the required width, purchase a custom “80%” lower, made for Thompson magazines, from , or fabricate a custom lower.  I plan to fabricate a custom lower, starting with a standard “0%” forging.

What changes are necessary to the feed ramp / barrel extension to work with a .45? My tentative plan is to grind the barrel extension, using a Dremel Sanding Drum
, to accommodate the .45.  I’ve done this for .458 Socom build. That’s just the plan though, and I shall delay execution until close to the end.  The angles and location will be very much determined by the other feeding factors that are TBD (To Be Determined).

Where should the magazine be placed for optimal feeding? This will have to be given some further thought.  Obviously the choice of magazine plays an important role in this consideration.  In an upcoming post I’ll model the relationship between the magazine and the barrel extension and determine how to best guide a .45 round into the chamber.

What additional considerations are there? It’s pretty typical to discover a few more as the design progresses, so I try to just accept for now that it’s likely to happen.  I’d like the new magazine to use the standard AR magazine release – this way my habit of reaching for that release is not frustrated by some other mechanism.  Of course that influences position of the magazine (consideration #5).  We’ll see what other desires crop up as we proceed.

With that much of a plan in place, I’ll start assembling the parts I know I’ll need:

  • 7.62×39 AR bolt
  • .45 chamber reamer
  • .45 chamber gauges
  • AR barrel extension
  • .451 barrel.  1 in 16″ twist.
  • Thompson magazines
  • Lower receiver “0%” forging.

Obviously I’ll also need an upper receiver, charging handle, bolt carrier, carrier key, firing pin, firing pin cotter, etc.  But the parts I’ve listed are those particular to this design.  While I wait for parts to arrive, I can start working out some more design details.

(to be continued…)

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