AR-15 Diagnostics

Most semi-automatic firearms lock the bolt or slide to the rear on the last shot. Here we’ll discuss what to do with an AR-15 that doesn’t.

by Joe Carlos

There are good reasons why we want the bolt to lock to the rear when the magazine has been run dry. It is a quick indicator of being out of ammo and can make reloading a bit faster. While some have advocated counting rounds, the reality is most people won’t in a fight. Military personnel sometimes use tracer rounds to indicate the magazine is almost empty but this isn’t a viable solution in many situations. It’s best to have the firearm work as intended.

So you’re not wasting time, some Stoner platform firearms don’t normally lock back on the final round. Pistol caliber and rimfire conversions usually don’t. However, it’s possible for an AR-15 that’s supposed to lock back on the last round function perfectly with a full magazine and fail to do so. Understanding the cycle of function is key to diagnosing such problems. One of the best resources about this is Army FM 23-9 M16A1, M16A2/3, M16A4 Rifle Marksmanship.

What we gunsmiths have to grasp is the difference in the bolt carrier travel necessary to reliably feed rounds from the magazine compared to the significantly longer travel required for the bolt catch to lock the bolt back on the last shot. This is because the flat surface of the bolt catch that traps the front of the bolt is located a fat 3/8” to the rear of the magazine. Anything that interferes with the carrier traveling that extra 3/8” can cause a failure to lock back. I can think of five primary causes of failures to lock back and an additional seven occasional causes.

Read more in the July 2016 issue.

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