Would a 300 Blackout subsonic with 30 inch barrel be quiet?

[ar15barrels]

Random question:

If you made a rifle chambered in 300 Blackout and used subsonic ammo and the rifle had a long barrel, say 30 inches, would this setup be quiet? Like hearing safe quiet? Silencer quiet? Would such a long barrel be a problem for the subsonic 300 Blackout?

Thanks for any info on this.

The pressure is going to be a LOT lower with a longer barrel which will certainly quiet it down a lot, but it probably won't get it to hearing safe levels.

Cartridge          : .300 AAC Blackout (SAAMI)
Bullet             : .308, 240, Sierra HPBT LR MatchK 9245 G7 Litz
Useable Case Capaci: 13.066 grain H2O = 0.848 cm³
Cartridge O.A.L. L6: 2.260 inch = 57.40 mm
Barrel Length      : 30.0 inch = 762.0 mm
Powder             : Hodgdon H110
Charge             : 7.4 grains = 0.48 grams

Estimates of the progress of combustion:
   NR.:   x(in.)     Z(%)    v(fps)   p(psi)    t (ms)
     0    0.00000    3.18      0.0      3626    0.0000
    104   16.32824   91.37    957.1      1773    2.1501
    116   29.33052   94.95   1048.3       943    3.2257
 Muzzle

The noise of a gunshot comes from the pressure being released at the muzzle as the bullet leaves the muzzle.
In the simulation above, the muzzle pressure is 943psi at the muzzle while in a 16" barrel with the same 1050fps velocity, the muzzle pressure is 2124fps.
Most of the lower pressure comes from more space to expand, but the shorter barrel also runs at a higher pressure to reach 1050fps than the long barrel does since the bullet continues to accelerate in the longer barrel.

[Milsurp1]

[hermosabeach]

bigger diameters tend to have just that- more area to burn powder

so a 45 caliber might be quieter given same bullet weight and velocity...



Friends have a 45 PCP air rifle.... just using compressed air it is loud without a suppressor.

[Starslinger]

The pressure is going to be a LOT lower with a longer barrel which will certainly quiet it down a lot, but it probably won't get it to hearing safe levels.

Cartridge          : .300 AAC Blackout (SAAMI)
Bullet             : .308, 240, Sierra HPBT LR MatchK 9245 G7 Litz
Useable Case Capaci: 13.066 grain H2O = 0.848 cm³
Cartridge O.A.L. L6: 2.260 inch = 57.40 mm
Barrel Length      : 30.0 inch = 762.0 mm
Powder             : Hodgdon H110
Charge             : 7.4 grains = 0.48 grams

Estimates of the progress of combustion:
   NR.:   x(in.)     Z(%)    v(fps)   p(psi)    t (ms)
     0    0.00000    3.18      0.0      3626    0.0000
    104   16.32824   91.37    957.1      1773    2.1501
    116   29.33052   94.95   1048.3       943    3.2257
 Muzzle

The noise of a gunshot comes from the pressure being released at the muzzle as the bullet leaves the muzzle.
In the simulation above, the muzzle pressure is 943psi at the muzzle while in a 16" barrel with the same 1050fps velocity, the muzzle pressure is 2124fps.
Most of the lower pressure comes from more space to expand, but the shorter barrel also runs at a higher pressure to reach 1050fps than the long barrel does since the bullet continues to accelerate in the longer barrel.

Hey thank you for that simulation! According to a couple of quick Google search, 116 decibels is about as loud as a leaf blower and the maximum safe exposure time without earplugs is about 20 seconds over an eight hour period. That actually might be hearing safe if you only take a few shots per day. I was hoping it would be quieter but still that's something. I feel a little less foolish for asking the original question.

[waveslayer]

Random question:



If you made a rifle chambered in 300 Blackout and used subsonic ammo and the rifle had a long barrel, say 30 inches, would this setup be quiet? Like hearing safe quiet? Silencer quiet? Would such a long barrel be a problem for the subsonic 300 Blackout?



Thanks for any info on this.

Not if the Velocity stays under 1050fps. That's the speed of sound and that's what makes the loud noise. So some subs may work, others may not with that length barrel. Just reload your own. If it's quite and has a Can it will depend on how good your Can is. Go TBAC if you want the best Can

Sent from my SM-G986U using Tapatalk

[pohorsky]

I've used cci 22 quiet and they are quieter than my air rifle in my 22 bull barrel. Still can kill things like skunks and other small animals.

[ar15barrels]

Not if the Velocity stays under 1050fps.
That's the speed of sound and that's what makes the loud noise.

The gas behind the bullet goes supersonic even though the bullet does not and the gas makes the supersonic boom.
Gas behind a bullet moves around 4-7 times faster than the bullet does as the bullet uncorks the barrel.

Look at 38 seconds here and you will see how much faster the gas is that the bullet:

In the first 1/4" of bullet travel outside the barrel, the gas is traveling at least 1.5" which means it's going 6x faster than the bullet.

A suppressor captures that supersonic gas and lets the gas out the front after it's expanded and is subsonic so it does not make a supersonic crack outside of the can.

[waveslayer]

The gas behind the bullet goes supersonic even though the bullet does not and the gas makes the supersonic boom.

Gas behind a bullet moves around 4-7 times faster than the bullet does as the bullet uncorks the barrel.



Look at 38 seconds here and you will see how much faster the gas is that the bullet:



In the first 1/4" of bullet travel outside the barrel, the gas is traveling at least 1.5" which means it's going 6x faster than the bullet.



A suppressor captures that supersonic gas and lets the gas out the front after it's expanded and is subsonic so it does not make a supersonic crack outside of the can.

Yes that is correct, but the bullet needs to be going less then the speed of sound making the Crack noise, so I was correct. You are correct as well, because it is two parts.

Ever shot subs versus supersonic? It's the bullet breaking the speed of sound at around 1050fps making the sonic Crack that is so loud. Suppressor acts like an exhaust system , cools the gasses so when they exit it isn't as loud. Shoot a Can with supersonic it is only treating the hot gasses, not the supersonic Crack of the bullet.

So like I said, it only matters what the Velocity is of the ammo. Shoot my can using subs versus supersonic, there's a big difference. Also first shot is always louder on some cans because of the cold air in them.



Sent from my SM-G986U using Tapatalk

[subscriber]

If you double the barrel expansion volume (length) the muzzle pressure drops to half. More actually, because the gasses also cool down faster after most of the propellant is consumed. That is very significant. It will cut the sound pressure by more than half.

Will you notice the sound reduction? Yes. It will be like shooting a .22 rifle compared to a .22 pistol. Will it be hearing safe? Probably not; even with subsonic loads.

By the way dB is a logarithmic scale. So, a reduction from 160 db to 140 dB is much more significant in absolute terms than a 120 to 100 dB reduction.

No. Just because the barrel is longer, isn't going to make the explosion noise dissipate. It might some, but I can't imagine in 20-30 decibel reduction like a suppressor does. Maybe like 2, and that's only because you are farther away from the muzzle, like standing two steps behind someone who is firing. It might actually be quieter doing what ^ mentioned, just using a blast shield/linear compensator on a regular barrel. Though I have a blast shield on a 10.5" blackout barrel, and I can't tell a noise difference between supers and subs (though I always wear earpro). Maybe a real long barrel you would.

[subscriber]

Idiot OP

That was not a stupid question. The problem is that the answer is subjective.

[subscriber]

Don't listen to the naybobs!

Do it!

Exactly. Do it. If it is not quiet enough you cap lop off the extra inches based on your own objectives.

[beanz2]

I seem to remember pics from a Scandinavian country where they were shooting through barrels open on both ends. The barrels were acoustic foam lined inside.

Does that help reducing noise to the shooter?

[subscriber]

I seem to remember pics from a Scandinavian country where they were shooting through barrels open on both ends. The barrels were acoustic foam lined inside.

Does that help reducing noise to the shooter?

It reduces mostly the sound travelling to distant neighbors. It will reduce sound reflected back to the shooter, if he were in or next to a concrete structure. Try shooting indoors without hearing protection to see what I mean. Better yet, don't...

Here is a sound tunnel made from a stack of tires: https://www.youtube.com/watch?v=c_oduKvZots&t=1352s