Problem with a 10 mm blowback AR

[jbs27572]

OK, some history here: I bought a TNW 10 mm PCC first. Recoil was mild; accuracy was decent but it jammed ALOT. It was very unreliable. When it did cycle, it only tossed brass about 6 ft (which was good). I spent hundreds at the local gunsmith trying to work out the kinks but finally gave up.

So, I ditched that one and bought a blowback AR in 10 mm from Pro2a. Accuracy is good and reliability is good too (in it's original configuration. However, it throws brass 30 yards hard enough to dent the brass as it hits the deflector. It came with a "heavy duty spring" and a 10 ounce buffer. the bcg is 14.5 oz so total reciprocating mass is 24.5 oz. The recoil is more jarring than that of the TNW too.

Things I have tried: I bought the strongest spring I could find (springco orange spring). Maybe a little better but very little if any. It still tosses brass into low orbit and dents the brass. I think the bolt is outrunning the magazine now though so it started jamming. I may need to go back to the original spring. If I could find a significantly heavier buffer, I would try that. I did find a 12.5 oz buffer available but it is $130 and I am not quite that desperate yet. I thought about taking the tungsten weights out of the buffer and filling with tungsten powder but I don't really know if I could get more mass that way. There is some wasted volume around the tungsten weights that would be filled but the space between the tungsten particles makes it have an apparent density of about 12 g/cc instead of the 19.3 g/cc of a block of tungsten metal (read that on the description of 300 mesh W powder on Amazon).

Any ideas on how to tame this beast? More mass would be awesome but I can't find a heavier bcg or an affordable heavier buffer. Everything is designed for 9 mm and 10 mm is a LOT stouter.

 

[Rosco1]

https://www.reddit.com/r/t5_3eg6f/s/5KarXaTdzm

This is place for any and all problems pcc related. KAK makes a replacement tungsten weight that goes in your bcg(out of stock at moment). The mod of the sub is a guru with any pcc problems

 

[jbs27572]

Thanks. I already called KAK and they don't know when they will get more of the tungsten bcg weights in as apparently, tungsten is hard to come by.

I have an extra long buffer tube coming today as well as a 15 oz buffer. I'll try that and see how much more weight I need.

 

[I_am_Bulletproof]

I had a TNW 10mm PCC that jammed, so I bought a Pro2A 10mm blowback AR. It's accurate but ejects brass hard and dents cases

 

[jbs27572]

I had a TNW 10mm PCC that jammed, so I bought a Pro2A 10mm blowback AR. It's accurate but ejects brass hard and dents cases

This was exactly my original post.

The problem turns out to be not enough bolt+buffer mass. 9mm needs 21 to 24 Oz reciprocating mass to function correctly; 10 mm is quite a bit hotter. Your P2A 10 mm bolt is 14.5 Oz and the buffer is 10 Oz. 24.5 Oz is not enough for 10 mm. You need 33 to 36 Oz reciprocating mass to tame the hotter 10 mm load. I had a friend machine me a spacer weight and a sleeve for the buffer to provide more spring compression to make up for the now longer buffer. I bought the 8.5" buffer tube from JR carbines to accommodate the 1.3" spacer and the Orange buffer spring from Springco. Now I have 33.6 Oz reciprocating mass and it has calmed down considerably. I am waiting for the tungsten bolt weight from KAK industries to come in and I will add that as well. I think it should come in around 35 Oz altogether. It still will cycle subsonic ammo as well as full power loads too.

 

[I_am_Bulletproof]

This was exactly my original post.

The problem turns out to be not enough bolt+buffer mass. 9mm needs 21 to 24 Oz reciprocating mass to function correctly; 10 mm is quite a bit hotter. Your P2A 10 mm bolt is 14.5 Oz and the buffer is 10 Oz. 24.5 Oz is not enough for 10 mm. You need 33 to 36 Oz reciprocating mass to tame the hotter 10 mm load. I had a friend machine me a spacer weight and a sleeve for the buffer to provide more spring compression to make up for the now longer buffer. I bought the 8.5" buffer tube from JR carbines to accommodate the 1.3" spacer and the Orange buffer spring from Springco. Now I have 33.6 Oz reciprocating mass and it has calmed down considerably. I am waiting for the tungsten bolt weight from KAK industries to come in and I will add that as well. I think it should come in around 35 Oz altogether. It still will cycle subsonic ammo as well as full power loads too.

Thanks, this was extremely helpful. I'll increase bolt and buffer mass. Could you share spacer thickness and measurements, buffer sleeve details, Springco spring model, and JR 8.5" tube fitment?

 

[jbs27572]

The steel spacer is 1.63" and contains two 1.5 Oz tungsten weights. It adds 7 Oz altogether. The sleeve fits over the 10 oz buffer (so it does not add to the length of the buffer) and weighs another 2 oz. The buffer tube is from JR carbines and has standard buffer tube threads to fit on an AR lower. Altogether, the buffer + spacer is 5.63 " long. This all fits in the 8.5 inch buffer tube with enough room for the bolt to get behind the bolt catch about 1/8 inches so it will still hold the bolt open. The spring is from Sprinco. Go to their website. Their springs are color coded. The orange spring is their strongest spring and is designed for 308 caliber and ultra short buffers. The steel sleeve just provides another 1.3" spring compression to approximate the spring compression of the original setup with a 4 inch buffer and a 7 inch buffer tube.