BCG Weight and specs

[MaineMan2]

I’m surprised by the variations in weight for bolt carriers for 308 ARs. Sons of Liberty has an LR308/SR-25 BCG that only weighs 12 oz. Yes, I realize that the SR-25 was a different firearm but it was designed by Stoner if I’m not mistaken. Also isn’t the LR308 the first DPMS pattern 308 AR?  JP Enterprises offers light weight, variable weight, and a “full mass” bolt carrier for 308 ARs. They state that the full weight version weighs more (15.31 oz. Excluding than a standard DPMS pattern. Aero Precision and Wilson Combat BCGs weigh about 18 ounces, but that includes the bolt itself, cam pin, firing pin, and cotter pin. 
 

After the rifle fires, isn’t it the combined mass of the BCG and the buffer what determines how fast the action will cycle?  If I’m correct there, then it seems you would have to compensate for a lighter BCG with either a heavier buffer or less gas. Am I missing something?  What is the weight spec for a “standard” DPMS bolt carrier or BCG?  JP specifically limits their low mass bolt carriers to competition shooting…

 

[98Z5V]

This will bring it all into perspective:

 

[MaineMan2]

Thanks.  I read that whole thread again and had missed the part where 98 talked about adding 4 oz to his bolt carrier to get proper function after changing buffer weights. 
 

I notice that people tend to talk about buffer weights without considering the spring. It too is part of the system and 98 refers to the original DPMS 308 having a special spring to make their system work. I’d already thought about this because we routinely experiment with different springs in semiauto pistols. You can’t change the reciprocating mass there…

[98Z5V]

Springs in pistols comes down to the ammo that you're shooting, and being able to make follow-up shots without excessive recoil and muzzle/gun jump.  And it's valid.  We really can't "tune" with spring in this platform, because there's a minimum needed, and most manufacturers don't meet that minimum, at all.  Some manufacturers think AR15 springs will work in these things, and that's just a drunken pipe dream, right there.  You can go too crazy on springs with these, and there's only two examples that I can even think of.  First is the Stag Arms .308 spring.  That's the heaviest spring I've ever seen in my life, first-hand.  That spring rate is through the roof.  I think they did that because they ship their guns with 3.8oz buffers.  I'm not willing to invest the time, research, or money, to even see if it's viable - but I've already done that for GunPusher John, on a customer's gun that came into the shop. I couldn't make that thing cycle reliably with all kinds of different buffers, including custom-lightened buffers.  I'm over it, with that Stag spring.

Second thing I can think of is the Sprinco springs, and accidentally getting the wrong one.  Sprinco makes the Orange spring specifically for the .308ARs with 7.000" receiver extensions, running short 2.500" buffers (that should weigh 5.4oz, or very damn close).  Specifically designed for that application.  Next is the Sprinco Red spring - that's a direct replacement for the Armalite EA1095 spring, which should be used in .308AR rifle recoil systems, and carbine recoil systems with a 7 5/8" extension, running a 3.250" 5.4oz buffer. 

If someone accidentally got a Sprinco Red spring, and threw it in a 7.000" AR-15 recoil system for their .308AR, they'd be Sad Panda.  It won't work.  Won't work better than putting the EA1095 into a 7.000" carbine setup, which wouldn't work, either... 

Therefore, we can only select the proper spring - and make sure the rest of the recoil system is straight, and make sure the gas system is straight.

I'll say this now, though - 8oz 9mm extended buffer, with a Sprinco Red spring, is what finally fixed my 9mm AR.  I need to get another set like that, and fix my first 9mm AR.  Blowback pistol cal ARs are a motherfucker to make run right, if you really, really are interested in making them run right... 

Edited March 11, 2022 by 98Z5V