Over that last several years I have had experience with several BCA uppers including two in 7.62x39. There is a high probability that it will have a minimum chamber that will not except some eastern european made ammo. Also they may need an 'enhanced' firing pin and and extra power hammer spring. One of mine was so overgassed the followers on some magazines with weaper mag springs would double feed. Switch the buffer for an H3 and it slowed down working with any AR 7.62x39 magazine. I had to do a bolt substitution to correct minimal head space with one gun.
A friend sent back his upper 5.56 because it would not not chamber military 5.56 of any caliber but might work with .223.
It may take some trouble to get an upper from them to work, but afterwards it should work well.
I purchased an upper in 7.62x39 because I have a lot of that ammo around.
It is a good deal except for what seems to be in my experience a 50-50 chance of insufficient head space. I fixed my head spaces issues with a new bolt.
Looking for a winter project. Im want to build an AR 15 in 7.62 x 39. MY Colt AR-15 A2s are perfect.
I want to build a perfect AR-15 in 7.62 x39. Who should I look to, for quality parts for my adventure?