Just got my stamp for my AR SBR. First thing to do is cut my 16" RRA 9mm barrel to 6" and attach my F1 silencer. I have had this can for 2 years now and has worked great on the 16". Car door quiet with 147gr, sonic crack with 115gr.

First shots are a lot louder than I expected. I thought the 16" was probably slowing the 9mm down and now the 6" was letting it creep above the sonic threshold. I cut barrel to 5" and tried again, still loud. 147 and 115 sound about the same.

Any advise? The 147 should be subsonic no matter what length???

Some 147 grain 9mm ammo can be transonic meaning it has the potential to go faster than the speed of sound. The HK MP5 is a prime example of such gun. The barrel on the HK is almost 9". Some 147 9mm will still crack with a suppressor installed while others will not. I suggest you chrono your SBR and see what speed your rounds are travelling. I'm guessing you're hearing the actual ignition of the round. Your longer barrel had a complete burn by the time the round entered the suppressor. What suppressor and ammo are you using?

jfk wrote:Your longer barrel had a complete burn by the time the round entered the suppressor.

Shorter barrel is going to have the bullet moving slower but you will have more incomplete combustion which is why they tend to be louder.

57fairlane wrote:

jfk wrote:Your longer barrel had a complete burn by the time the round entered the suppressor.

Shorter barrel is going to have the bullet moving slower but you will have more incomplete combustion which is why they tend to be louder.

That was what I was guessing. Damn, I really liked how quiet the gun ran but wanted to have it a short as possible. It is still quieter than a 22LR but you can tell it is a gunshot now.

you have 1/2 the pressure at the muzzle at 16" than you do at 5"

I think your reloading bench and a Chrono are going to be your best friends.

I'm guessing your not using K baffles.

Capt. Link. wrote:I'm guessing your not using K baffles.

No, M baffles. A no-lathe build.

twodollarbill wrote:I think your reloading bench and a Chrono are going to be your best friends.

Exactly. The most effective and efficient path to quiet shooting goodness with the hardware at hand is to work up a load (or find a commercial load) that does what you want. Three elements here; the gun, the can and the ammo. Work the ammo angle.

PostalGlock'n wrote:

Capt. Link. wrote:I'm guessing your not using K baffles.

No, M baffles. A no-lathe build.

Possibly the baffles could be ported to respond to the higher pressure.Often the phenomenon is seen in reverse when K baffles are used and a rifle will be louder than a pistol.