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btmj · August 25, 2013, 09:49 PM

MarkCO is clearly speaking from experience. Everything he says makes sense to me.

Speaking from as a fatigue engineer with 25 years experience working with military platforms... hear are my thoughts on the BCG...

Some of the testing aspects of the Mil-Spec are simply to save the military the need to test fire the component. For example, a high pressure test of the BCG followed by MPI is designed to reveal manufacturing flaws that would lead to a short fatigue life. However, you or I could achieve the same thing by running 500 rounds or so through the rifle. Any initial flaw would reveal itself by then. Once a bolt has 500+ rounds on it, it is no different than an MPI'd bolt. Both have been tested to reveal inherent flaws in the steel. For the military, it makes sense to inspect the bolt with an MPI technique. They don't have the luxury of firing hundreds of rounds at the range.

Shot peening the BCG is an odd practice in my opinion. Rarely is shot peening called out as part of an original design. It is usually a Band-Aid to address a fatigue problem that comes up in the field, but in my experience, the military never allows shot peening to be the only solution to a problem. They will demand a substantive fix, and the engineer will usually have to "show it good" without the shot peen. So I would bet money that at some point there was a fatigue issue with the bolt, and a design change was made... could have been a change in material or heat treat, could have been a change in geometry... and the engineer (probably at Colt) had to prove the design change fixed the problem... Oh and by the way, they threw on some shot peen for good measure. It's like belt AND suspenders.

The shot peen needs to be done before the proof testing. It is a fatigue no-no to shot peen a surface that has been exposed to proof loads. If a crack was started by the proof loading, the peening will not make it go away, but it will make it impossible to find. The crack will likely still propagate as a sub-surface crack. So peening needs to be done before any loading.

Carpenter 158 is an interesting choice for a steel bolt. It is a plastic molding steel... It is used to make die cast or injection molds for plastic parts. Evidently it works pretty good for rifle bolts as well. But don't ascribe any magical properties to it. It is just a good quality tool steel. Nothing more nothing less.

The oddest (and most bizarre) feature of the mil spec BCG is staking of the gas key fasteners. "staking" is a low tech and unsophisticated way of providing anti-rotation of fasteners. There are a lot better ways to do it. You won't find very many staked fasteners in a modern automobile engine, and you will never ever find them in aviation. A high temp thread adhesive would work much better in my opinion. But the one thing that staking has going for it, it allows a government inspector to know at a glance that those two fasteners have been staked. And I strongly suspect that this is exactly why those two bolts are staked... it makes the anti-rotation feature inspect-able.

If you have an AR with a non-mil-spec BCG, my advice is to put 500 rounds through it, remove the BCG and clean it up and put it in a Ziploc bag. Then put in a new BCG and put 500 rounds through it.... Now you have a known reliable BCG in the gun, plus a spare that you know to be reliable. The BCG represents half or two-thirds of the likely field problems an AR might have (other than magazine issues), and BCGs are super easy to replace. You are good to go.

August 25, 2013, 09:49 PM	#22
btmj Senior Member Join Date: November 1, 2011 Location: Near St. Louis, Missouri Posts: 864	MarkCO is clearly speaking from experience. Everything he says makes sense to me. Speaking from as a fatigue engineer with 25 years experience working with military platforms... hear are my thoughts on the BCG... Some of the testing aspects of the Mil-Spec are simply to save the military the need to test fire the component. For example, a high pressure test of the BCG followed by MPI is designed to reveal manufacturing flaws that would lead to a short fatigue life. However, you or I could achieve the same thing by running 500 rounds or so through the rifle. Any initial flaw would reveal itself by then. Once a bolt has 500+ rounds on it, it is no different than an MPI'd bolt. Both have been tested to reveal inherent flaws in the steel. For the military, it makes sense to inspect the bolt with an MPI technique. They don't have the luxury of firing hundreds of rounds at the range. Shot peening the BCG is an odd practice in my opinion. Rarely is shot peening called out as part of an original design. It is usually a Band-Aid to address a fatigue problem that comes up in the field, but in my experience, the military never allows shot peening to be the only solution to a problem. They will demand a substantive fix, and the engineer will usually have to "show it good" without the shot peen. So I would bet money that at some point there was a fatigue issue with the bolt, and a design change was made... could have been a change in material or heat treat, could have been a change in geometry... and the engineer (probably at Colt) had to prove the design change fixed the problem... Oh and by the way, they threw on some shot peen for good measure. It's like belt AND suspenders. The shot peen needs to be done before the proof testing. It is a fatigue no-no to shot peen a surface that has been exposed to proof loads. If a crack was started by the proof loading, the peening will not make it go away, but it will make it impossible to find. The crack will likely still propagate as a sub-surface crack. So peening needs to be done before any loading. Carpenter 158 is an interesting choice for a steel bolt. It is a plastic molding steel... It is used to make die cast or injection molds for plastic parts. Evidently it works pretty good for rifle bolts as well. But don't ascribe any magical properties to it. It is just a good quality tool steel. Nothing more nothing less. The oddest (and most bizarre) feature of the mil spec BCG is staking of the gas key fasteners. "staking" is a low tech and unsophisticated way of providing anti-rotation of fasteners. There are a lot better ways to do it. You won't find very many staked fasteners in a modern automobile engine, and you will never ever find them in aviation. A high temp thread adhesive would work much better in my opinion. But the one thing that staking has going for it, it allows a government inspector to know at a glance that those two fasteners have been staked. And I strongly suspect that this is exactly why those two bolts are staked... it makes the anti-rotation feature inspect-able. If you have an AR with a non-mil-spec BCG, my advice is to put 500 rounds through it, remove the BCG and clean it up and put it in a Ziploc bag. Then put in a new BCG and put 500 rounds through it.... Now you have a known reliable BCG in the gun, plus a spare that you know to be reliable. The BCG represents half or two-thirds of the likely field problems an AR might have (other than magazine issues), and BCGs are super easy to replace. You are good to go.