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Posted: 3/12/2011 5:40:24 AM EST
I'm thinking about getting a gas piston system retro fitted to my 7.62x39 Colt. I do own AK-47s including a very nice Norinco with miller receiver, and I own several AR-15s in a variety of calibers and configurations. I understand and appreciate the strengths of each platform.

Reading about piston system recommendations on the forum, I'm currently considering the Adam Arms kit but open to other suggestions. On adamsarms.net, they indicate:

"Currently our system will function with the 5.45x39, the 5.56/.223, the 6.5 Grendel, and the 6.8 SPC without any special modifications needed. "

I just emailed them regarding compatibility with 7.62x39. While waiting for their reply, please let me know what you think. I welcome more feedback on the AA kit and any other suggestion. Thanks.
Link Posted: 3/12/2011 10:11:59 AM EST
I bought an 11" Model 1 Sales 7.62x39 upper a few months ago and swapped it over to an AA piston kit. With that length of barrel and the very small dwell time after the gas hole it would not cycle to save its life. I sent it down to Ron Williams and discovered that they already figured out 11" and piston don't get along. I had him cut it to 7", which the have figured out works with a piston set up, and should be getting it back next week.

I'm not sure on the 16"+ side of the house, but it is a lower pressure round and needs some dwell time for the piston to receive and good bit of pressure to fully cycle the action.

My .02

Eric
Link Posted: 3/12/2011 1:32:25 PM EST
I asked Osprey about x39 and thier kits. The AFRcom rep said it is GTG. I installed mine but haven't shot it yet. Only drawback is it's hard to dissassemble for cleaning if you use corrosive ammo.

Posted Via AR15.Com Mobile
Link Posted: 3/13/2011 6:26:51 AM EST
[Last Edit: 3/13/2011 6:27:36 AM EST by ar15seeker]
Thanks. I guess the pressure level is depended on the distance between the chamber and gas port on the barrel. Chances are if AA does not work on your 11", it won't work on my 16" CAR.

How about the Ares Defense kit? Same problem?
Link Posted: 3/13/2011 6:44:16 AM EST
The way it was described to me by Ron and Greg is that the x39 is a low pressure round when compared to the 5.56. This, combined with very little barrel (pressure vessel) after the gas port causes the gas pressure reaching the face of the piston to be diminished. The 7" takes care of this by having a "pistol length" gas port location which puts more pressure to the piston face sooner.

Eric
Link Posted: 3/13/2011 8:05:01 AM EST
[Last Edit: 3/14/2011 2:19:03 PM EST by ar15seeker]
Originally Posted By towerofpower93:
The way it was described to me by Ron and Greg is that the x39 is a low pressure round when compared to the 5.56. This, combined with very little barrel (pressure vessel) after the gas port causes the gas pressure reaching the face of the piston to be diminished. The 7" takes care of this by having a "pistol length" gas port location which puts more pressure to the piston face sooner.
Eric

Thanks Eric. That totally makes sense. Pressure = Force / Area. With 7.62, the surface area inside the barrel is greater than 5.56, even with the same force from the expanding gas, the pressure would be lower. With the 7" pistol barrel, by reducing the distance between the chamber and gas port, the area is reduced, thus higher pressure level.

With 16" CAR, I hopeful the 5" over the 11" barrel would allow the round to seal in and maintain the pressure a little longer, maybe long enough for the piston to work (lower and lower pressure as the round traveling farther down the barrel and increasing the surface area).
Link Posted: 3/14/2011 11:59:49 AM EST
Originally Posted By ar15seeker:
Originally Posted By towerofpower93:
The way it was described to me by Ron and Greg is that the x39 is a low pressure round when compared to the 5.56. This, combined with very little barrel (pressure vessel) after the gas port causes the gas pressure reaching the face of the piston to be diminished. The 7" takes care of this by having a "pistol length" gas port location which puts more pressure to the piston face sooner.
Eric

Thanks Eric. That totally makes sense. Pressure = Force / Area. With 7.62, the surface area inside the barrel is greater than 5.56, even with the same force from the expanding gas, the pressure would be lower. With the 7" pistol barrel, by reducing the distance between the chamber and gas port, the area is reduced, thus higher pressure level.

With 16" CAR, I hopeful the 5" over the 11" barrel would allow the round to seal and maintain the pressure a little longer, may long enough for the piston to work (lower and lower pressure as the round traveling farther down the barrel and increasing the surface area).



That was my thought exactly once Ron showed me the light so to speak. I think you should be fine with a 16" set up. If I remember correctly, Mr. Williams said something like a .90" size gas hole in the barrel for a 7.62 piston set up...but I could be misremembering and you may want to check with him.

Eric
Link Posted: 3/15/2011 7:52:32 AM EST
Black Rifle Arms is supposed to be releasing a 7.62x39 piston system.
Here are a couple links of some prototypes:
7.62x39 piston system
Low Profile Prototype
Link Posted: 3/15/2011 2:40:23 PM EST
Originally Posted By towerofpower93:
Originally Posted By ar15seeker:
Originally Posted By towerofpower93:
The way it was described to me by Ron and Greg is that the x39 is a low pressure round when compared to the 5.56. This, combined with very little barrel (pressure vessel) after the gas port causes the gas pressure reaching the face of the piston to be diminished. The 7" takes care of this by having a "pistol length" gas port location which puts more pressure to the piston face sooner.
Eric

Thanks Eric. That totally makes sense. Pressure = Force / Area. With 7.62, the surface area inside the barrel is greater than 5.56, even with the same force from the expanding gas, the pressure would be lower. With the 7" pistol barrel, by reducing the distance between the chamber and gas port, the area is reduced, thus higher pressure level.

With 16" CAR, I hopeful the 5" over the 11" barrel would allow the round to seal and maintain the pressure a little longer, may long enough for the piston to work (lower and lower pressure as the round traveling farther down the barrel and increasing the surface area).



That was my thought exactly once Ron showed me the light so to speak. I think you should be fine with a 16" set up. If I remember correctly, Mr. Williams said something like a .90" size gas hole in the barrel for a 7.62 piston set up...but I could be misremembering and you may want to check with him.

Eric


I got the Adams Arms system to work in 7.62x39 from 14.5" & 16" barrels at the Carbine Gas location.
Neither barrel would work with the piston kit in the mid-length position.
Anything shorter than 14" & I would use the pistol-gas location as it works fine down to a 7.5" barrel.
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