Smyth Busters: Can You Mix & Match Bolt-Carrier Groups?

One of the great things about the AR-15 platform is its interchangeability of parts. This includes the ease of swapping a different bolt-carrier group into your rifle, which takes all of about 20 seconds. But some folks out on the Internet say, "Don't mix and match AR-15 bolt-carrier groups!" Why not? Let's ask Steve and Caleb.... who say AR-15s are all manufactured to the same specs, so a new bolt-carrier is almost certain to fit in your rifle. Just whip out the headspace gauges and check the headspace on the new BCG. You don't even need a full set of headspace gauges. Just a NO-GO gauge is fine. (Don't use a "field" gauge: that's for military and law enforcement users who put a very large number of rounds through their rifles during training.)

Here's another warning about mixing and matching bolt-carrier groups: Let's say you have two BCGs that both headspace correctly in your rifle. You set one of them aside and shoot thousands of rounds with the other. Don't just swap the "spare" BCG into the gun without checking the headspace again. The lug recesses in the barrel extension have worn while you were shooting with the other BCG, and the spare bolt-carrier group may no longer headspace correctly. How many rounds lead to significant wear? That varies by rifle, type of ammo, gas system pressure, and a lot of other factors. Suffice it to say, the average casual shooter is probably not going to significantly wear the headspace on their rifle any time soon.

So the myth is BUSTED, with a stipulation. You CAN change your AR-15's bolt-carrier group IF you check the headspace when you do it.