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Suppressd 300BLK louder than it should be

I've had a 16" 300blk for a couple years. I use it in conjunction with an old 18T AAC 762-SD suppressor. Some subsonics are quieter than others, but it's always been quiet.

I just built a 300blk pistol and took it out to test function and found it to be really loud. I had 3 flavors of subsonic ammo and they all seemed to be going supersonic. I was at about 1000ft elevation, it was cold and rainy, and for life of me I couldn't find any reason why that would drop the speed of sound.

A single round out of about 20 I fired came out with the quietness I've come to expect out of subs.

I tried both my 8.3" pistol and my 16" rifle. It seemed all rounds were going supersonic.

I thought perhaps the suppressor has died somehow, but I've never shot 22lr through it, and only a couple mags of cast bullets.

Any ideas? Anyone with an 18T suppressor want to go shooting and see if mine works with yours and yours works with mine?



Elevation, temperature, barometric pressure, all can change the speed of sound.

I’ve witness 300 blackout factory ammo go super when it was labeled sub, not an uncommon thing. I’d pay close attention to the advertised FPS.

Another factor is higher pressures in the shorter barrel. Shooting any caliber on a longer barrel tends to result better suppression than on a shorter barrel. The pressure curve for longer barrels tend to be lower than shorter barrels, resulting in less muzzle signature for the suppressor to tame.

In other words, yes the shorter barrel should be louder.

Why the subs are going super, beats me. Could have been the weather.


if it went super and it's supposed to be sub... then it was made for a shorter barrel or they tested it some other way.

Not much you can do if it's supersonic other than burn through it as supers.


My 8" SBR sounds load to me but if I let someone else shoot it or hold it at arms length its way quiter. also have to use a fast burning powder like lilgun.


Reno is correct, temperature is usually the biggest factor. The colder it is, the lower the speed of sound.

Reno also mentioned the higher pressures in shorter barrels, which can also be affected by gas system length. If your 16" barrel has a carbine-length gas system but your pistol barrel is a pistol-length gas system, there's another possible factor.

The biggest question I have is regarding port pop. Are you using an adjustable gas block, and have you reduced the gas any? AR pistols tend to be loud by their nature, but tuning the gas system can often help when using a suppressor.
Port pop from ar. Heavier buffer. Adjustable gas block. Barrel length is the biggest factor. You want quiet, go longest barrel as you feel you can, go bolt gun. Or ar platform tuned proper. I’m building a 10.5 blackout now. I know it’s going to be loud. My 556 stuff is soo much louder on my short barrels.
Your plan to use a chronograph is a good one. Whn developing supressed sub reloads in my 7.5" barrel, I noticed significant lower sound when the velocity dropped below 1000fps compaired to 1050 or so. And several of the factory loads I tried were on the threshold and sounded like supers. This with gemteck tracker 30 cal can. Good Luck
100% on the Crono testing.

Also if you're loading your own ammo, you might have a new batch of powder that is behaving differently than previous jugs of the same stuff. This would be really apparent if your loading right below the speed limit. A new batch of powder with a little more density, hotter mix etc. and pressures can go up.



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