Muzzle Devices for 13.7" Barrel

[H3nchman]

You can put a big spacer behind a muzzle device when the muzzle device does not have a big counterbore or chamfer in the threads.
The chamfer removes a few threads from the brake so that the brake can thread all the way back on a barrel which does NOT have the last few threads against the shoulder.
Surefire has a significant chamfer.
KAC does not have such chamfer.
That's why that spacer is made for a KAC.

When you COMBINE a brake with a big thread chamfer with a spacer that pushes the brake away from the barrel shoulder, you are removing more and more threads of engagement.
The Surefire brake's chamfer removes about 3 threads, similar to a crush washer.
A normal A2 flash hider WITH a crush washer or a Surefire with minimal alignment shims will have around 14 threads of engagement.
1/4" spacer is taking away 7 threads of engagement.
Then you need some clocking shims to align which would take away another one, leaving you 5-6 threads.
The engineers say that 3 threads is absolute minimum for a threaded junction but most designers want twice the screw diameter of engagement length.
Of course we don't have 1" of screw engagement on ANY 1/2-28 threaded AR barrel since the usable threads are only 1/2" long after the crush washer.

Thank you for the explanation. I learned a lot. That makes sense.