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Old January 26, 2020, 11:12 PM   #269
P Flados
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Join Date: January 8, 2017
Location: Wilmington NC
Posts: 96
Stag, I understood what Mehavey has been talking about since the beginning, but did not focus on it as we were trying to understand the cause of the overpressure.

First, lets assume that the bolt has the correct depth for a recess. A bolt problem big enough to matter would be gross and obvious.

For an AR, inadequate case wall support is caused by the way the barrel is cut, a bad barrel extension, how much chamfer/rounding was done or a combination of the above. The associated dimensions and tolerances are generic to all AR-15 platforms that use a standard bolt and barrel extension (including the 223, 5.56, 300 BO, etc.). There is nothing about this that would be different for a 350L.

I have cut and threaded a couple of AR barrels. The distance from the working face of the barrel extension locking lugs to the back surface of the barrel needs to be just enough for a bolt to rotate into the locked up position. With a barrel out of the upper, insert a bolt and rotate it into the locked up position. It should have very little free play forward and backward. I took a spare barrel and the free play was about the thickness two layers of card stock.

Next there is any chamfer and/or rounding at the chamber entrance. The removed metal for a chamfer and/or rounding increase the effective distance from the bolt face to the back of the chamber.

If there was no free play and no chamfer/rounding, the only portion of the case not fully enclosed by the chamber would be the portion extending into the bolt face recess.

If your free play of an engaged bolt is more than it should be, you could have a problem even with a normal chamfer / rounding at the chamber entrance. If your chamfer / rounding is excessive you can be in trouble even with normal free play. If both are "on the long side" it could be the combination that causes the problem.

Excessive free play is just bad machining by the guy that cut the barrel (most likely) or by the maker of the barrel extension (less likely).

Limits on the chamfer / rounding may not be as explicit and/or may be harder to measure/control. With all of the "my gun does not feed" complaints that the 350L had early on, cutting too much chamfer / rounding could have been intentional by someone that did not fully appreciate the downside of what they were doing.

Last edited by P Flados; January 26, 2020 at 11:40 PM.
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