35x39 subsonic gas port info

[nekshot]

I have a 21 inch 35 cal, .357 barrel I am fitting to upper. It has no gas port and I am using a x39 cartridge opened up to 35 cal with a suppressor as the cartridge. What would be best gas length for suppressed ?

[tdbru]

nekshot,
from everything i've read so far, when i was building my 6.5Gr AR from parts, it seems most recommend a rifle length gas system if at all possible. now i assume you're going to drill a gas port? and if you're going to go to that effort and don't mind several trips on the drill press, i'd start with a very small hole. and it the rifle cycles, done. if not, open it up a few thousandths and try again.

the soviets did a 9x39 for their Spetsnaz teams using suppressed rifles. sounds like you're trying to re-create that. do a search on 9x39 and AR and see if anyone before has already been there, done that.

have fun,
-tdbru

[Klem]

nek,

I take it you also mean subsonic and not just a suppressor on the end of a supersonic rifle. Let's face it, a suppressor is only half the noise equation - with a calibre like that you are not wanting the bullet to break the sound barrier, yes?

If that is the case I would go a pistol length gas port.

Using Blackout as an analog the trend has been to shorten gas port distances from carbine length to pistol length over the years. You will also not find a Blackout AR barrel longer than 16" because for subsonic it is not needed and becomes problematic the longer the barrel is. 16" is a compromise length in that calibre between fast supersonic and subsonic. If you just want to shoot subsonic only it is more efficient to go even shorter.

The reason for going a short gas port length in an AR is that peak pressure subsides quicker as the calibre gets wider. Your .35 cal is wider than Blackout .30 cal and so your barrel pressure will drop even quicker with all that volume to dissipate the gas behind the bullet as it travels down the pipe. As for barrel lengths, longer barrels make it harder to keep the bullet subsonic AND cycle the action. Longer barrel accelerate the bullet; more inches = more velocity. So if you then load less powder to keep it subsonic you end up with less pressure at the port to cycle. You need at least 4,500lbs of pressure at the port to cycle an AR, using the lightest MILSPEC buffer and spring. In Blackout a lot of rifles won't even cycle subsonic unless they are suppressed/ Suppressors keep that pressure up for a little longer. Getting that pressure high enough at the port to cycle is critical. Unlike other calibers and supersonic ammo you won't have the luxury of overpressure to tame. A subsonic round in a Blackout is only 2/3 the SAAMI peak pressure limit and your calibre will be no different.

If your project was a bolt gun this would be easier. In an AR however your powder needs to generate pressure at the port but then have the bullet exit high subsonic, the most efficient being 90fps below the speed of sound wherever you are. Which is why commercial subsonic ammo for all calibres is around 1010fps. The supersonic crack start building at 90fps below the speed of sound and continues getting louder until 150fps over the speed of sound. The quietest it will even be is 90fps low. Any lower and you are robbing yourself of range and impact energy.

You also have to watch that you load is not below 80% of available case space. Below this is dangerous, risking Secondary Explosion Effect and Flash Over.

For subsonic-only something a lot shorter like a 12" barrel would be more efficient. 21" is way too long for what you want to do, plus the dwell time between a gas port at 4" and the gas bullet exiting and gas dropping at 21" will be a problem.

Unless you don't want subsonic ammo and then it gets a lot easier with 21". Given the larger diameter pipe I would go carbine length for .35cal, supersonic only.

[nekshot]

thanks every one for your input. Klem, I was totally ignorant to the need for a shorter barrel with a suppressor. I really don't need any more supersonic barrels but I do want to play with sup sonic loads.

[nekshot]

nekshot,
from everything i've read so far, when i was building my 6.5Gr AR from parts, it seems most recommend a rifle length gas system if at all possible. now i assume you're going to drill a gas port? and if you're going to go to that effort and don't mind several trips on the drill press, i'd start with a very small hole. and it the rifle cycles, done. if not, open it up a few thousandths and try again.

the soviets did a 9x39 for their Spetsnaz teams using suppressed rifles. sounds like you're trying to re-create that. do a search on 9x39 and AR and see if anyone before has already been there, done that.

have fun,
-tdbru

Yes, I saw a write up on that cartridge and a military man shooting it and the gun he was shooting looked like a sweet unit to play with.