A very common question we get here at Veriforce Tactical what are the threads are on my AR-15 barrel. Well there is a few ways to tell, Here is a list of the most common thread pitches per your Caliber and the likelihood of your AR barrel being these threads:
5.56 – 1/2×28 threads (always)
.223 – 1/2×28 threads (always)
.308 – 5/8×24 threads (always)
7.62×51 – 5/8×24 (always)
.300 blackout – 5/8×24 (usually, but rarely 1/2×28)
6.5 grendel – 5/8×24 (usually, but sometimes 1/2×36 and 9/16×24)
6.8 Creedmoor – 5/8×24 (always)
6.8 SPC – 5/8×24 (always)
.224 Valkyrie – 1/2×28 (always)
9mm – 1/2×36 (usually, but sometimes 1/2×28)
7.62×39 – 5/8×24 (usually, but sometimes 1/2×36)
50 beowulf – 49/64×20 (always)
.458 Socom – 5/8×32 (usually, but sometimes 5/8×24 and rarely 11/16×24)
.450 bushmaster – 11/16×24 (always)
This is a list of NORMAL thread pitches per the caliber, now some manufactures will choose to make an odd ball thread pitch for their barrel. Thats usually because they will have an abundance of one kind of pre-done muzzle brakes so they make it fit. Or they just don’t want to spend extra money on new taps or programming. You should always check with your manufacturer if you can. If you are in the market for AR-15 muzzle devices check us out for the biggest selection of USA made muzzle brakes and flash hiders with our easy to use product filter to get to your muzzle brake or flash hider faster.
Here is a good way to tell if you think your barrel is 1/2×36, but don’t want to spend money on one just to test. Most of us have a 5.56/.223 rifle with a flash hider or muzzle brake that is 1/2×28 threads; use it to test the barrel. start to thread it on, you should make it about 1.5 turns before it gets stuck. If that happens then you should have 1/2×36 threads. If it goes on all they way, you have 1/2×28 threads.
Why would manufactures threads a .30 caliber bullet with .20 caliber threads pitch?
Usually due to cheapness or ignorance. The manufacture will have a bunch of 1/2×28 flash hiders lying around, so instead of making/ordering new ones at 1/2×36 (as in the case of 9mm rifles) they will just thread the barrels at 1/2×28 threads. Its lazy by our standards and CAN be dangerous.
Is it dangerous to thread use .20 caliber threads on .30 caliber bullet?
Yes and no. If you are using flash hiders then you are fine, they are open at the end and don’t port gasses for muzzle compensation. Muzzle brakes are dangerous if you have smaller threads. You cannot use them unless they are drilled out to the proper opening. In fact in recent years we leave extra space in our opening for just this reason.