Here is a good page describing the essential components and design considerations in an AR-15 action. In this post I summarize some research I did focusing on the tail end of the system: The recoil spring and buffer. In order to see exactly what goes on in there I cut a viewport into a buffer tube, clamped rifles into my test fixture, and recorded high-speed video of the action cycling.

My primary motivation was to see whether changes in the buffer tube area could improve an AR’s ability to run “undergassed” loads optimized for subsonic shooting. One of my hypotheses is that the best subsonic loads use light charges of fast powders: light charges make less noise, and faster powders should produce more consistent muzzle velocities, which are essential to the accuracy of subsonic projectiles at ranges past 50 yards. The problem with light charges is that they don’t produce a lot of gas volume, and it’s the pressurized gas tapped off the barrel that drives the action in most modern autoloading rifles. It’s easy to work up .300BLK loads with fast powders that push 220gr bullets to the 1000fps subsonic sweet spot, but that don’t generate enough gas to fully cycle the action: If the bolt doesn’t travel far enough rearward, it will fail to eject the spent case (FTE) and/or fail to feed the next round in the magazine (FTF). So: If we’re reducing powder loads and reach a point where the gun experiences failures to cycle, can we get it running again by lightening that spring or buffer weight?

And while we’re at it … Look at the top load in that first video above: The bolt is running so hard that it’s slamming into the rear of the buffer tube. Wouldn’t the gun run better if we tuned the buffer system so that the bolt just barely reaches the rear of its travel?

It turns out that when the bolt slams the buffer into the rear of the tube there is a bit of a rebound effect that increases its return speed and energy, cutting cycle time by 5-10ms. We can see this effect in the following video showing the same buffer configuration running on decreasing amounts of gas.

If reliability is your primary concern you’d probably prefer to err on the side of running the system harder to reduce the chances of a FTE or FTF. (However, this only works up to a point: I’ve run autoloading actions so hard that they get consistent double-feeds. Not certain why … maybe the bolt slamming into the rear flexes the receiver frame or jostles the magazine enough to pop a second round into the path of the bolt.)

Now, let’s hold the round we’re shooting constant and instead see what happens when we switch to a lighter spring or a lighter buffer: Turns lightening either of those components makes the action run faster:

In the course of this study I inadvertently illustrated one more feature of the AR buffer: I created the “light” buffer for these tests by completely removing the weights from a standard buffer, so it’s actually an empty buffer. This resulted in the bolt bouncing back visibly when it hit the breech, which is not a good thing as it could produce a failure to fire in the moment the bolt is bouncing (since it has pulled away from the case, possibly far enough that the firing pin can’t get a solid strike on the primer). Turns out the floating weights in the buffers serve to deaden the impact against the breech, preventing that bolt bounce.

Back to the original question: The next video shows that if we lighten the spring we can get a short stroking load to run a full stroke. However, the lighter spring slows the return to battery, and that lower return energy could increase the odds of a FTF (which can become a problem with tight magazine springs or underlubricated bolts).

What about lightening the buffer? Aggregating across the data I collected, it’s clear that a lighter buffer speeds up the cycle (by reducing inertia) but it can’t increase the stroke length enough to get a short-stroking action to run reliably. Here’s a good side-by-side comparison of four variations: