Optimal 9mm AR barrel length?

[wahoo95]

147gr HST stays subsonic out of my 5" TROS.

One of my favorite SD loads for my AR9mm is 147gr XTP over 6.5gr of 3N38 for 1,180fps from my 5" TROS!

[DDLES]

147gr HST stays subsonic out of my 5" TROS.

One of my favorite SD loads for my AR9mm is 147gr XTP over 6.5gr of 3N38 for 1,180fps from my 5" TROS!

1,180fps is transonic. You will notice a significant drop in sound signature when you are around 950fps.

http://en.wikipedia.org/wiki/Transonic

"Transonic is an aeronautics term referring to the condition in which a range of velocities of airflow exist surrounding and flowing past an air vehicle or an airfoil. Air flow velocities are concurrently below, at, and above the speed of sound at the pressure and temperature of the airflow of the air vehicle's local environment (about mach 0.8–1.2). It is formally defined as the range of speeds between the critical mach number, when some parts of the airflow over an air vehicle or air foil are supersonic, and a higher speed, typically near Mach 1.2, when all of the airflow is supersonic. Between these speeds some of the airflow is supersonic, and some is not."

[wahoo95]

1,180fps is transonic. You will notice a significant drop in sound signature when you are around 950fps.

http://en.wikipedia.org/wiki/Transonic

"Transonic is an aeronautics term referring to the condition in which a range of velocities of airflow exist surrounding and flowing past an air vehicle or an airfoil. Air flow velocities are concurrently below, at, and above the speed of sound at the pressure and temperature of the airflow of the air vehicle's local environment (about mach 0.8–1.2). It is formally defined as the range of speeds between the critical mach number, when some parts of the airflow over an air vehicle or air foil are supersonic, and a higher speed, typically near Mach 1.2, when all of the airflow is supersonic. Between these speeds some of the airflow is supersonic, and some is not."

Yes I knew that, sorry I wasn't clear but I wasn't saying that particular load was subsonic or quiet cause it's far from it....it makes a nice SD load though I do try to keep my Subsonic loads around 950fps.

[DDLES]

Gotcha, sorry about that.

[rob_s]

Interesting chart DDLES. Looks like no real velocity benefit to barrels longer than about 5" with that particular load, and as you said after about 4" there may be louder sound.

You said you're doing 4" barrels? Is that a 4" threaded barrel? I have a YHM 9mm can that mounts to a flash hider.

[DDLES]

Interesting chart DDLES. Looks like no real velocity benefit to barrels longer than about 5" with that particular load, and as you said after about 4" there may be louder sound.

You said you're doing 4" barrels? Is that a 4" threaded barrel? I have a YHM 9mm can that mounts to a flash hider.

Yep, threaded 1/2X28

[SuicideHz]

I had no problem keeping reloads subsonic out of my colt 10.5" upper.

Reloading is the way to go. 147gr. rainier bullets and iirc, 4.3 grains of red dot...

[duncan]

If you reload with a slow burning powder like HS-6 or Power Pistol, you can get a 115 grainer up in the 1500 + fps range for 75-100 yard shots.

With quality 115 or 124 grain ammo, you should get a 16" barrel.

Find some GECO or MFS South African 115 grain ammo - nice and hot and will shot long accurately.

If you use PMC or some other crappy ammo, you should still be getting another 75-100 fps with the longer barrel.

45ACP does not accelerate much after 10.5". That is why 45ACP is best suited for AR-15 pistols - 9-10" barrels. Having ADCO cut mine down soon

[m4fun]

I fire a lot of suppressed 9mm - been making my own ammo for quite a while now - 147gr and 4.8gr Unique powder. More or less tailored for me to works well suppressed barrels up to 10.5"

[LonghunterCO]

I had read that the Uzi was built around the 10" barrel because anything longer than that, the bullet actually started slowing down...looking at the cart that doesn't seem to be the case.