I know that they are mostly for show and do very little besides look cool.  Especially out of a short barrel.  But is it safe to assume that a comp used for 7.62 x 39 is ok to use as long as the threading matches?  I'm looking for something to fill about an inch or two  of hand guard while I wait on getting a suppressor.  There is very little out there designated for 9mm and I see some people using 7.62 brakes.  Thanks for the help.

I'd make absolutely sure a 9mm will fit through the bore of the muzzle device. If it won't, it'll need to be bored out.
I'm 99% sure a PWS Triad for 7.62 will pass a 9mm.

So just because its designated 7.62 doesn't necessarily mean a 9mm round will pass through?

7.62 is a .308

9mm is .355 dia

7.62x39 most common barrel threads are 5/8x24 tpi

9mm is 1/2x28 tpi or 1/2x36 tpi and even some more that are less common

BUT DO NOT use a x39 brake on a 9mm without reaming it open to .41-.42

Think about it. Trying to stuff a .354 - .356 projectile into a 30 cal hole. not safe

drill it to .370 - 3/8" drill bit. You can see the muzzle just inside the f/hider or comp once drilled . BUT ALWAYS. Make sure there is enough clearance, then some for the round to pass through w/out issues.
I have been using the YHM 6.8 flash hider from brownells   they are 1/2 x 36 . I did have to remove some off the back of the hider to index correctly.  Buying a used lathe was one of the better investments we've made. It will never pay for itself monetarily, but does pay when you need something and it's just a walk to the shop to do it.

http://www.brownells.com/rifle-parts/muzzle-devices/flash-hiders/phantom-5c2-comp-flash-hider-prod24940.aspx?psize=96


100-010-688WB
Flash Hider 6.8mm, 7.62x39, 9mm

Mfr Part: YHM805C2

Ok.  I understand now.  Thanks for the responses.

I used an A2 style 1/2 x 36 TPI.
Got from an internet search, $15 IIRC.

They protect the threads when the can is off,