AR15 Buffers - 300 BLK

Gidday - I'm having a head scratch. Currently building up an AR, in 300 Blk. Barrel is 12.5" with a pistol length tube, I've fitted a adjustable gas block and with the "factory" Formosan buffer it will cycle and lock back with 1.5 turns open on the AGB with a 125 Gn "supersonic" load, 18 gns WAP296, which gives 2030 fps. Its definitely as far as you'd want to go in this rifle. Brass was landing about 4.00 o'clock.

Anyhoo I wanted to eliminate the buffer "twang" - had enough of that right under my ear in the Territorials, so I got a Armaspec stealth buffer from NZAR. The only one they had was an H3, I got it figuring I could always put it in the lathe and reduce the weight a bit. Much to my surprise it cycles just fine on the same 1.5 turns open setting of the AGB without any modification.

Any thoughts on this result??

I'm finding "Gassing" for AR's to be a bit hard to get my head round despite watching lots of Youtube and reading even more.

There are a multitude of potential reasons, and the answer is probably a little bit of all of them.

An M16 BCG with a carbine buffer weighs around 14.6oz. Swapping out a 3oz carbine buffer for a 5.4oz H3 buffer is only increasing the total reciprocating mass by roughly 16%.

If you consider the gas impulse to the carrier to be fixed (it’s not, but this will give us the worst case scenario for BCG velocity and energy drop), a 16% increase in mass would reduce the bolts velocity and kinetic energy by 13.8%.

The majority of ARs (outside of highly tuned competition guns) will be ‘over-gassed’ by more than 13.8% to ensure cycling in adverse conditions and with low powered ammo, so an H3 buffer isn’t going to reduce the bolts energy enough to make them choke.

The energy stored in a spring increases with the square of its compression, so even if an AR was setup so that the bolt only just cycled, a 13.8% reduction in bolt kinetic energy won’t cause the bolt to short-stroke by 13.8% (in terms of bolt travel). It depends on the spring rate, preload and design, but it will be much less than this.

The Armaspec Stealth uses a dual spring setup, so it will store substantially more energy in the final stages of compression compared to the initial stages. That means, given a certain kinetic energy reduction, there should be less of a short-stroke with the Armaspec compared to a standard single spring setup.

Delayed unlocking and extraction reduces friction by giving more time for chamber pressure to subside and for the brass casing to cool and contract, meaning less of the carriers kinetic energy is used to complete the unlocking and extraction operations and more energy can go towards the other stages of cycling.

The gas impulse the carrier receives isn’t constant with different carrier/buffer weights, even if the gas setting remains the same. Gas pressure acts on the carrier for a fixed distance (the length of the cam track). A heavier carrier/buffer experiences a lower acceleration and therefore takes longer to reach the end of the cam track. The length of time gas is pushing the carrier rearward is increased, consequently increasing the impulse delivered to the carrier. As a heavier BCG/buffer experiences a larger impulse, the reduction in kinetic energy is slightly offset.

Also, maybe the spring in your Armaspec is just weaker than your old spring and none of the above really matters. :thumbsup:

Hey thanks @Banana for such a comprehensive and informative reply, that totally makes sense to me, I haven't seen anyone explain it as clearly as that before.