Building an AR-15 in .458 SOCOM

The .458 Socom compared to the .223 - the two use the same magazine.


There are several choices for those wanting a big bore in an AR-15.  The .458 Socom uses the ubiquitous .458 bullet – the same bullet used in the .458 Winchester – so there exists a great selection of suitable rifle bullets for it, ranging from 300-600 grains.  This made it an attractive selection for me.  I’ve already discussed how to chamber a barrel and assemble an AR-15 upper reciever so I won’t repeat that here.  I’ll just cover the differences that are specific to the .458 Socom.

The .458 Socom was designed with the AR platform in mind.  As such, the rounds fit in the existing magazines, and no modifications to an AR-15 lower receiver are required to accommodate it.

The upper receiver is a different story…

Obviously a barrel chambered in .458 Socom is needed.   I won’t repeat barrel chambering instructions , except to note that you can feed a .458 reamer deeper between passes while chambering, because the reamer flutes are deeper.

Applying fresh cutting oil to the reamer for another pass.

The barrel is cut in the typical manner, and fitted for a standard barrel extension.

Checking the headspace using the barrel extension, bolt, and feeler gauges

The barrel extension, however, must be modified.  Using a Dremel-type tool, the feed ramp is ground at approximately the same angle as the feed ramps present on a factory barrel extension.  I used the <a href=”″>Dremel Sanding Drum</a><img src=”″ width=”1″ height=”1″ border=”0″ alt=”” style=”border:none !important; margin:0px !important;” />
to grind mine.  I ground a little at a time to prevent heat build up and soon had a feed ramp wide enough for a .458 bullet.

Just getting started - you can see how grinding has begun, on the lug between the .223 feed ramps

Grinding continues with plenty of pauses to prevent heating the part

The completed barrel extension. Note that the grinding doesn't have to blend the new ramp into the .223 ramps - that would be taking off more material than necessary

The bolt is a modified 7.62 x 39 bolt.  I’ll detail the modifications in my .45 ACP AR-15 posts,  (Coming Soon!) but the bolt must be opened up to accept a .308 or .45 caliber rim and the extractor must be modified to suit.

The only other modification to make is to the upper receiver, particularly the ejection port.  I set the upper receiver in a multi-angle vise and set it up so the angle was the same as the existing angle at the bottom of the ejection port.  I opened the ejection port 0.080″, in the “Y” axis, from the standard port.  By opening up only the bottom of the port, a factory dustcover may still be installed, if desired.

Enlarging the ejection port to accomodate the big .458

The gas system is set with a 0.125″ gas port at carbine length on the barrel.

The resulting rifle works great and I can feed 350 grain Hornady bullets loaded to an OAL of 2.07″ through stock magazines.  Heavier bullets may not work in all magazines.

Other than that, it’s just another AR-15 build.  This is about as straightforward as an odd-caliber AR build can get.

The completed rifle. It still needs a finish, but was range ready at this point. It shot surprisingly well, with only a single failure to feed, and that round was well out of spec and would not chamber under any conditions


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One Response to Building an AR-15 in .458 SOCOM

  1. Pingback: Designing a Gas-Operated Pistol Caliber AR-15. | Firearms Designer

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