Is it possible to get a subsonic 308 to cycle in an AR-15?

[happy7]

My friend was talking to me, wondering if he can get a mold to cast a bullet heavy enough to cycle a subsonic in an AR platform chambered in 308. I have my doubts about it, but he is interested in it for short range and so doesn't really care if it tumbles. Does anyone know how heavy the bullet would need to be? Has anyone played with this idea?

[MtGun44]

HMM. I assume you mean an AR-10 or variant, since it is impossible to get a .308 length
cartridge to function in an AR-15 action or magazine.

In any case, you will need to experiment with different powders and boolit wts. Probably a
slow powder and heavy boolit could possibly provide the port pressure that you need.

Bill

[happy7]

You are right. I guess it shows that black guns are not my thing. But my friend is into them, and I do want to help him if I can.

[popper]

Some have opened the port a few thou and added an adjustable gas block to allow full power shooting also. Some have had luck with the 300 blkout CB, others have not. Definitly needs a carbine bbl. You can get the gas to work but twist and accuracy may need some work. I've shot 170's 30-30 CBs with light but NOT subsonic loads, gas works but accuracy @ 50 isn't particularly good. It really shouldn't be any different then the blkout, you just have a higher volume case and may need some filler.

[Lizard333]

It isn't going to be easy. In order to get the velocity below the 1080 FPS threshold to keep your boolit sub-sonic, you at reducing your pressures significantly. I have tried with my RR AR, M1A, and other variants to get the actions to operate. You are limited in your powder choices as well. You want the powder to burn, not detonate.

I have had great success using Clays powder in my 223, but not getting it to cycle the action.

The other double edge that has to be skated is accuracy of the sub-sonic load. You may find one that operates the action, stays under 1080 FPS, but won't hit the broad side of a barn.

Let us know what you find.

[rockrat]

It all depends. I would think that for your scenario to work, you might get by with a carbine barrel and port or a barrel with a port designed for an SBR (short barreled rifle). In the AR15, there are barrels with ports in different locations, depending on your barrel length. There are rifle, carbine, and pistol ports and also ports with different diameters.

If your friend has a carbine barrel, he might get it to work. If he has a longer barrel, probably not, IMHO. Unless, he sends the barrel in and gets another port drilled in the barrel. If he is running a free float tube, he could put a fake gas block over the rifle port and use a gas block/gas tube over the carbine location, like Bushmaster does with their 450 Bushmaster 20" barreled upper. Use an adjustable gas block, so when he runs full power ammo thru the 308, he can cut down on the gas flow so as to not batter the upper.

[Adam10mm]

Easiest way to get an AR to cycle with subsonics is with a suppressor mounted, so the backpressure of the gas being restricted in the suppressor drives the action. When you're using a suppressor on the AR, it increases the cyclic rate of the action due to more gas being delivered into the system than unsuppressed. The excess gas being delivered out the muzzle, but with the suppressor the gases are slowed and retained in the weapon for a longer period of time, thus more gas gets into the gas tube. I've seen this on my demo M16 full auto. Put the suppressor on and the rate of fire increases dramatically.

The biggest issue with subsonics is stabilizing the bullet. Paper test before attaching the suppressor. Contrary to popular belief, the bullet isn't perfectly stabilized when it exits the barrel and that's the main reason for the baffle bore sizes being what they are.

What I do is try a bunch of different powders and loads that get subsonic (1030-1050fps) and then paper test them at 10-15 feet to make sure they are stabilized. Take the subsonic stabilized loads, attach the suppressor and test to see if they cycle. If not, back to the drawing board.

I used to have a list of how much thrust or back pressure was needed to cycle the action on certain semi-auto weapons but I lost it when my previous computer crashed. It was pretty easy to figure a load out using QuickLoad and calculating the port pressure of the load.

[popper]

I went through the numbers a while back, seems like I remember the number as being 1/10 peak pressure for 308.
subsonic (1030-1050fps) the typical number used, the can gives the sound suppression. Check regs forthe can lic., cost and cleanable can/lic. problems.

[Adam10mm]

1050fps is the speed of sound at sea level at 32 degrees. If it's warmer, the velocity is higher. 1050 is suggested as a standard because it will allow the bullet to remain subsonic with the suppressor boost which adds 20-30fps typically.

I'm very well versed in how suppressors work and what they actually do, thanks. It's also not a license for a suppressor, it's a transfer tax. Big difference.

[Artful]

Can it be made as a subsonic only shooter - yep

http://www.ar15.com/forums/t_3_121/460470_subsonic_308_AR_10.html


I drilled out the gas port on my 16" AR10 & put a switchable gas block on it. The gas length is considered Armalite's AR10 Carbine mid length, which is about 0.20" longer than a normal AR15 mid-length gas system, so it is pretty long for subsonics.

I emptied the buffer & installed a Wolff reduced power buffer spring & it would almost cycle, but the BCG would stop over the mag, just before closing.

I had the older-style bolt carrier with the flat bottom. Their newest one is beveled/sharpened it so it will slide over the cartridges in the mag with less resistance. I ordered the newest generation & it cycled. I think I may have trimmed the buffer spring until it would cycle.

I ordered the 4-5 different subsonic .308 loads that Midway sells & they all worked fine. The extreme shock was Much louder than everything else for some reason.

I never shot it for groups or handloaded anything for it ..... too many other projects I guess. The 7.62x39, or .300 Whisper are much more practical & don't have as big of a problem with having too much case capacity.

I got Dave to CNC some reduced capacity brass cases once upon a time - internal volume about the same as 7.62x39 or 300 blk. You can chase a dream and sometimes you can catch it - but do you understand it once it's caught.

POF AR-10 shooting sub's
http://www.youtube.com/watch?v=dgiMdwvNc_w

[Artful]

I used to have a list of how much thrust or back pressure was needed to cycle the action on certain semi-auto weapons but I lost it when my previous computer crashed. It was pretty easy to figure a load out using QuickLoad and calculating the port pressure of the load.

Is this what you lost?

http://www.orions-hammer.com/blowback/

or this?

http://www.scribd.com/doc/31569868/Sub-Machine-Gun-Designer-s-Handbook

http://www.dtic.mil/ndia/2010armament/WednesdayCumberlandPhilipDater.pdf
or are you looking for this?


A formula for determining what impulse level you're looking for is to multiply nominal velocity times the bullet weight. Then divide the weight bullet you're contemplating into the product and the figure you get will be the velocity needed to get the same energy impulse.
IE: 1,900fps X 110gr = 208,999.98
208,999/155gr = 1,348fps

[popper]

freakshow10mm - Other readers may not have your expertise. There are state differences to what can be used and the cleaning vs removablity problems. An old oil filter could be used. There is a difference between subsonic and supressed.

[Adam10mm]

Yes, subsonic is below the speed of sound. Suppressed is the sound being attenuated by a device such as a sound suppressor (also known as a silencer, moderator, or muffler). A subsonic or supersonic load may be suppressed or unsuppressed. Supersonic suppressed is not as quiet as subsonic suppressed.

[happy7]

Thanks for the replies. We will continue with this project and if we have any success, I will post the details.

[garym1a2]

Ar15 type subsonic try 40sw with180gr and load it down.