I have a couple of AR barrels that i want to cut down by 1/2 inch (for reasons).  The threads are the standard 5/8 inch long with a slight relief cut at the base of the threads.  If I have the barrel cut down by 1/2 inch, the new threads will essentially have a relief cut at the base and at the muzzle end.  The crush washer will take up the base relief cut, so that's not an issue, but will the the remaining ~0.375-0.400 inches of threads be enough for a flash hider?  I am attaching a rough sketch of what the resultant threads will look like - new cut on top and original on bottom:

Thanks!!

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You don't need that large of a start relief. You don't really need it at all. Just do a full circumferential chamfer, on the start. Also, you can eliminate the crush washer and gain thread length. Just Rocksett the muzzle device and use a precision shim if timing is necessary.

The threads are already cut with a relief cut at the shoulder (~0.125") with a 0.625" tenon length.  So there is approx. 0.5" of actual threads.  If I cut 0.5" from the overall length, the shoulder relleif cut would then become the relief cut at the muzzle end.  Whether there is a relief cut at the shoulder now is moot, as the crush washer will take up about 0.125".  So I am left with essentially 0.375" of treads holding the flash hider.  Is 0.375" of threads sufficient?

 Thanks!

8 to 9 threads of engagement on properly cut threads is strong enough for a flash hider.

“Reasons”?  Going for that “seamless” suppressor mount look at the handguard end I bet.  The squareness of the shoulder(indicating off the bore) and re-crowning of the muzzle are the critical machining steps. Fixturing the workpiece in the lathe is where the payoff is, the cuts are trivial.

I hope not w/ a crush washer.

In the world of engineering 3 full threads is the rule of thumb for 80% hold strength. The benefit of additional threads is diminished for each additional thread. However, the extra threads on a muzzle device provide additional bearing surface which is helpful when things will be threaded and unthreaded more often.

If you're putting this muzzle device on and leaving it I wouldn't give it a second thought, it's no problem.

If it will be removed just be more cautious.