I am in the process of getting my SBR barrel threaded to put a suppressor on it. It’s a Bushy AR, 7.5 inch, .223 1:9 twist barrel and on another post someone said that I should be careful about putting a suppressor on such a short barrel. And something about short barrels do not stabilize the bullet and that you can get a baffle strike. I was surprised to hear this directly relating to the length of the barrel not the twist to bullet weight.

What is the story? Is this true? Am I out of luck with putting a suppressor on my gun? It is too damn loud without a suppressor.

In a 7.5" barrel with 1x9 twist, the bullet will not even make 1 full revolution before it leaves the bore. It could very possibly be unstable, causing baffle strikes. Longer, boattail bullets will be less stable in this situation than shorter, flat base bullets.

So would it be safe to say I would be fine shooting 55grain bullets?

I still have some difficulties understanding the problem.

Regardless of the number (or lack) of revolutions the bullet is still rotating at 1 turn per 9 inches. So at the end of a 7.5 inch barrel it has rotated 300 degrees. The barrel, regardless of length, has imparted a 40 degree per inch twist on the bullet.

That said, I just don’t get it.

The shorter the bbl is, the higher the pressure is at the muzzle.  Phil Dater (PHD here) of GemTechdoesn't recommend anything less than 11.5" for cans.  That's a nice number anyway, because it still gives you some fragmentation of M193 at short range.  The problem, as I understand it, is that slight imperfections in the muzzle can lead to bullet yawing on very short bbls because of the greater pressure as the bullet exits the muzzle.  Coupled with a lower velocity (and thus lower rotational velocity), you have a potentially unstable bullet entering a can.  Bad JuJu.

I went with a GemTech M4-02 on a 11.5".  It's not "quiet" (139 dB) but it's shorter than a 16" with an A2 FH and will save my hearing if I ever have to use it indoors.

Thanks that makes sense. And nice gun.

There is 2 issues to using M193 bullet with suppressor...

Excess heat over time will supposedly cause the lead inside the jacket to become molten, so the bullet will do goofy things ..

Take a std M193 bullet, look at it, there is minimal bearing surface for the rifling to cut.. bullet looks like a football ---> Take 1 and set it in the jaws of a caliper, similar to how it would ride in the barrel, and look at the actual contact with the surface of the caliper jaw.   Your safest bet with a small holed suppressor is to use a premium bullet with a flat bearing surface.... I used a lot of Hornady, 52gr HP, and 55SP from Speer.  Most of the "good" bullets have a straight flat brg surface, so stability should be no issue.

That said, I have also used std M193 out of a BOWERS "MOUT",  on my 7.5" DPMS, and had no issues with the small slight over 1/4" exit hole.

The MOUT has been changed to aluminum guts  recently, and resides on a 10/22, as it actually too small diameter and too short for killing muzzle blast on 556.  My main 556 suppressor now is a WARRIOR,  which replaced my Triple-X as a repair/upgrade.

So it sounds hope is not lost. What I have in a 97S carbon 15 pistol that I would like to put a Gemtech M4-02 piranha on. The Gemtech was selected because it is of small enough diameter to fit in the handguard and thread fully on the barrel. A bonus is that it will look real nice with only the suppressor being visible coming out of the handguard.

So it sounds like I could get away with it if I selected my ammo carefully, or do you not think so?  

I don't believe the bullet will stabalize.  I would try a tighter twist like 1:7 or get an overbored suppressor.  I'm using an M4-2000 bored for 7.62 in part so I don't have to worry about using a short upper and getting baffle strikes.  I believe most manufacturer's won't waranty a suppressor on any barrel less than 11.5 inches.

I would ask Gemtech, and see what they say. I am pretty sure that they will give you a straight answer.

Nice rig Fizassist.

BEWARE....