I'm still willing to bet that he's undergassed for some reason. Block alignment, gas port issue, tube placement, loose gas key, bad gas rings (unlikely but always worth a check). If he was moving too fast, it would be acting the same as it does with a suppressor.....ie....trying to jam a 2nd round down the pipe before it can eject the 1st one. That would sound like a more "classic" cyclic rate issue. Sounds like the carrier doesn't have enough Hmmmph. Which is strange for a carbine length in the first place.