AR9 muzzle threads

[kotchz]

I have to ask this instead of thinking logically because believe it or not I was once wrong. I was told that the threaded foxtrot barrel was 1/2x36 which sounded reasonable. Removing the muzzle device I found the fit between the two to be unreasonably sloppy. I want to put a suppressor on the muzzle so I needed to find out where the problem was. I have nothing in that thread to try for fit. I measured the major (diameter) of the threaded part on the muzzle and found it to be .489. This was done with a vernier caliper. FM products say that this is correct. I know that 1/2x28 is .500 or close. So am I wrong here. Doesn't make sense to me. I do know that if I buy an adapter for my can and it's that sloppy I'll probably be in the market for a new barrel.
What do you guys think and don't say that Foxtrot is junk please.

[s4s4u]

Could be the MD has loose threads. No way to know for sure until you get the adaptor for the can and try it. 36 TPI is pretty much SOP for 9mm tubes.

[kotchz]

What I am asking does a 1/2x36 threaded muzzle actually measure .500?

[s4s4u]

One would think so, but with threads the major diameter isn't the critical dimension. Take Acme threads for instance. Pitch diameter and engagement between the male and female counterparts is what is important. With two parts from different manufacturers, even though the same size and TPI, you can have stacking tolerances and some will mate tighter or looser than others. As far as major diameter, there is an allowed tolerance depending on the thread class.

[1168]

I just measured a 1/2x28 (Green Mountain) at .495 and a 1/2x36 (unknown brand) at .492. However, my calipers are cheapos. Hopefully someone that knows the answer will come along.

As far as slop, I think shoulder dimensions are probably more important. Do you have an alignment rod?

[Steve Shannon]

Here’s the spec published by Ballistic Advantage:

https://dealers.ballisticadvantage.com/blog/wp-content/uploads/2016/06/1-2-36-thread.pdf