Low pressure cycling of the AR10

D

Donald

Guest
Any of you gunsmiths or others have any experience in getting a AR platform to cycle shooting subsonic rounds? The rifle has a legal suppressor on it and am looking for information on what would need to be changed to get the rifle to cycle with the low pressure of the sub sonic rounds. Any info or links would be appreciated.

Donald
 
lighter spring...try an ar15 or a 9mm spring.

larger gas port....

try shooting it like a 308...and deal with the results......
( as in shoot sonic...let the supressor workm but you will have a "crack" still.
your position is secure, the bullet is heard as it goes by)

mike in co

ps
try ar15.com
 
Rifle in question belongs to a friend that hunts hogs. He has spent the time and money to get the AR10 (.308) legally suppressed. He has a load that was developed by the smith that shoots a 170 grain bullet at about 1100 fps. He said it made a lot less noise than a 22 RF. But it will not cycle the bolt, which does not suprise me. Pressure appears to be in the 20,000 psi range +/-. I suggested a lighter spring but hesitated in suggesting he enlarge the gas port in the barrel as he may want to switch back to regular 308 loads. Anyone messed with a setup like this? Ideas welcome, as I know very little about the black rifles, other than they all appear to be pretty much alike. Just not my cup a tea.

Donald
 
Try heavier bullets. But I really believe he is going to have to treat it as a straight-pull.

Somebody who was really worried about quiet wouldn't want the action to cycle anyway. Makes too much noise.

Dual-purpose with automatic functioning isn't really an option with a full-bore cartridge in a silenced automatic. If he wanted that, he should have gone with a low-powered cartridge like a .30 Whisper that has a much lower velocity potential.
 
The case being used in this AR10 is the full sized 308. It is loaded to less than 1100 fps by using about 8 grains of Trailboss powder with the 170 grain bullet. I broke one of the rounds down and weighed the components. The load of Trailboss comes about 2/3 up in the case. Therefore there is no sonic crack from the bullet and the suppressor takes care of the muzzle blast. He may just have to use it as is and cycle it my hand. Thanks anyway guys. I have put him in touch with SSK Industries that builds the Whisper line of cartridges.

Donald
 
I've been toying with a similar situation. That of getting a full. locked breach. AR style bolt/carrier system to operate with 22lr. What I was loking at was a "piggyback gas system" using a CO2 source as the cycling gas in a piston system instead of direct inpingement. The light cartridge charge would trip a poppet valve, allowing a prechaged gas charge to cycle the action. It's still in the concept phase, no prototyping work done yet.
 
Vibe said:
I've been toying with a similar situation. That of getting a full. locked breach. AR style bolt/carrier system to operate with 22lr. What I was loking at was a "piggyback gas system" using a CO2 source as the cycling gas in a piston system instead of direct inpingement. The light cartridge charge would trip a poppet valve, allowing a prechaged gas charge to cycle the action. It's still in the concept phase, no prototyping work done yet.
Click to expand...

why reinvent the wheel..???..they make drop in conversions or complete uppers in 22lr.

mike in co
 
mike in co said:
why reinvent the wheel..???..they make drop in conversions or complete uppers in 22lr.

mike in co
Click to expand...
Did you miss the part about wanting a full locked breach lockup? Exactly who is it that makes a 22lr drop in that is not a simple blowback action? And the conversion to 5.7x28? - also with a locked breach? Who are this "they" ?
Whould a 22lr drop in be of any help to the OP?
 
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Like I said what I know about the AR system you could put in a thimble. But it appears to me that just using a weaker recoil spring in the 308 system would solve the problem of cycling, assuming the gas pressure would unlock the bolt. I may however be going down the wrong road. I assume in a normal system the gas pressure unlocks the bolt and then assists in recoiling the bolt assembly. Working in the dark here....

Donald
 
The muzzle energy of a subsonic bullet increases linearly with the bullet weight. The BC of the bullet determines how fast it loses that energy. The only way to get decent subsonic downrange energy is to use heavy low drag bullets.

The energy of any bullet increases with the square of the velocity and so does the energy it takes to drive it. There will be proportionally more energy available to operate the gas system with the square of the muzzle velocity. A 308 downloaded to subsonic velocity will have no more than a quarter the energy from the standard gas port and maybe as little as 1/10 as faster powders will be needed engrave the bullet and still give acceptable muzzle velocity with the excessive case capacity of the 308.

You could possibly get it to work by adding a second gas port at about half (or less) the distance from the chamber with a second gas block and a valve to switch between them, but you'll still end up with an unnecessarily heavy rifle with too long of barrel and (likely) poor velocity SDs causing excessive vertical stringing. Barrel friction is significant in subsonics and can't be ignored. The twist of most AR-10s is too slow to stabilize heavy subsonic 308 bullets. You'll also have the problem of fitting a 308 cartridge with heavy bullets in an AR-10 magazine which defeats the whole purpose of having a semi-auto. There's the additional problem that short barrels are better for subsonics and long barrels better for supersonics. A 16" to 20" barrel may work but it's not ideal for either.

For a subsonic AR-10 a second upper in either 458 Socom or 300-7mmBR would my recommendation. Both have 308 bolt faces and can feed heavy bullets from an AR 10 magazine with minor modifications. The 458 Socom can shoot 500+ grain bullets and gives decent subsonic muzzle energy. The 7mm-338 can shoot 300 grain SMKs and gives the best long range energy, usable to 500+ yards limited only by vertical dispersion.
Both of those of options would require an additional upper receiver and a custom barrel but they'd give better results than in 308.

A solution with little gunsmithing? Keep the 308 AR-10 unmodified for supersonic use and for subsonic shooting use a 16" 50 Beowulf AR-15 carbine with 600 grain RBT soft points. It requires no modifications other than (maybe) threading the barrel for a suppressor. For under 1/2 the cost a 45-70 Marlin lever action shooting 600 grain cast lead would do as well.
 
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Items to examine

Donald,

You are correct in your assumption of the gas system operation and of course it also cocks the hammer. Items to look at, for you, are carrier weight, buffer weight and action spring length. You don’t mention barrel length, port diameter or location, so the items above are the things to examine. Some carriers are substantially heavier than others and will delay unlocking too long for your light pressure rounds as will a heavier buffer. By reducing the weight of both of these components and shortening the action spring one coil at a time, you may get the bolt to cycle correctly.

I'm assuming that the bolt is at least unlocking and starting back. If it's not doing that minimum amount, then your friend needs to build a dedicated upper for subsonic use - moving the gas port aft and increasing its diameter.

Please ensure the gas port is not partially covered by the gas block - it wouldn't be the first time that's occurred.

Going to the heaviest/longest bullet possible with his present twist at the same velocity, will require more powder, which will increase port pressure. This will help cycling.

Another area too look at is the carrier's finish, on the bottom. Some are rough as a cob and will retard bolt movement. Ensure that the portion of the hammer that the carrier strikes is polished, as is the carrier.

I built a 6mm PPC for a gent who insisted he wanted the gas port 4 inches up the barrel from the normal location. Rough calculations indicated that this was going to cause cycling problems, because the port pressure was too low. By employing the techniques outlined above and going with a gas port diameter of 0.1065 (normal for a 223/220/6mm is roughly 0.0935) I was able to get it to function satisfactorily. The cycle time could be measured with a calendar, but the recoil from a rapidly cycling bolt was dramatically reduced – which was his original goal.

Good luck,

HPC
 
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HPC,
Thanks very much for your insightful information on this subject. The rifle my friend has is a box stock AR10 and the rounds he shoots were developed by the gunsmith that set him up with the paperwork to get the suppressor. And they do shoot and the bullet, 170 grains, is stabilized. A longer bullet, I suspect would not stabilize, in fact I am suprised the 170 does at this low velocity. I have also informed him of the low energy generated, kind of like a pistol round. I think it calculates out to about 370 foot/lbs. But he is satisfied with everything except the cycling. I will pass your information on to him. Thanks again..

Donald
 
mike in co said:
he has the opposite problem...too little pressure, not too much. full power loads with a suppresor is what armalites system is used for...he is shooting lite loads.

mike in co
Click to expand...

My thinking was to open up the port diameter to increase pressure for subsonic loads, as others mentioned, and then use the "selective gas system kit" to achieve normal operational pressures for factory loads.
 
Don said:
My thinking was to open up the port diameter to increase pressure for subsonic loads, as others mentioned, and then use the "selective gas system kit" to achieve normal operational pressures for factory loads.
Click to expand...


ok...not a bad idea....
the real problem is the owner....he wants too much from one gun.

mike in co
 
mike in co said:
ok...not a bad idea....
the real problem is the owner....he wants too much from one gun.

mike in co
Click to expand...

Mike,
I think you are right. Asking too much from one platform. Thanks for all the input. Your time and thoughts are appreciated. Time to end this thread I think.

Donald
 
Since it is the suppressor that is the NFA item and not the rifle, I believe the best course would be to obtain a AR15 and a .300 Whisper upper threaded for the suppressor. That would be more practical than modifying the suppressor for other calibers.
 
try 8.5gr Red dot with the 175gr

Need to use a 190gr Sierra or a 220 grain.

have to raise the pressure to get the action to cycle but gas volume small and fast enough only to obtain subsonic velocity.
 
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