Back when cheap 7N6 5.45x39mm was first made available, there were not many AR15 options out there.
My first configurations consisted of using standard 5.56 bolts and having the bolts bored out .400″ to handle the 5.45×39 case head. For barrels, I had Bulgarian chrome lined 5.45x39mm barrel barrels head spaced for the AR platform as seen below:
I had a Michael’s Machine AR-47 lower and grinded on the top of the mag catch which allowed for the proper mag height. I got a spare catch so I could go back to using standard 7.62×39 AK mags.
I thought that doing a piston setup would be ideal for 5.45×39 but it didn’t work out.
Pictured below is an old Ares GSR-35 drop in piston kit and the piston rusted in the cylinder rendering the action inoperable.
I also tried the Adams Arms piston kit and it suffered the same issue of the piston rusting and seizing up in the cylinder.
This is a video of that configuration in action before it seized up and the op rod got bent.
Then MGI came out with an AK-74 specific mag well, I prefer this since it has the familiar AR style push button release instead of the AK flapper style. Also notice that I’m using a PSA KS-47 bolt carrier which is clearanced for unmodified AK-47 mags but works fine for AK-74 mags.
Cyclic Rate testing
Surefire OBC modified to clear unmodified AK74 as well as AK47 mags and drums. 11.5″ chrome lined Bulgarian barrel headspaced for AR platform. Gas port .081″. Kynshot hydraulic RB5005, 556 Tubb Flat spring and spacer. Custom ‘fat boy’ gas tube.
Same upper but now using PSA KS-47 carrier and 5.45×39 bolt. Fat boy gas tube again. 556 Tubb spring and RB5007.
8″ 5.45×39 upper. PSA KS-47 carrier. RB5007, 556 Tubb flat spring and spacer.
8″ 5.45×39 upper. Surefire OBC 685 / 697 RPM. RB5005, 556 Tubb flat spring and spacer.
8″ 5.45×39 upper. Milspec carrier with 9mm Colt weight installed. 534 / 541 RPM. RB5007, 556 Tubb flat spring. Note that I did try the 9mm Colt weight carrier in the 11.5″ and it was too heavy to cycle properly with the .081″ gas port.