Only Cemetery would be crazy enough to work up a load for the AR-15 in black powder. Just keep scrolling. Looks like the black doesn’t quite have the energy to cycle the action, but I just think it’s great to try it at all.
UPDATE: Looking at this a bit further, it looks to me like the black powder isn’t even providing the recoil system with enough gas to unlock the bolt. I’m wondering if a heavier bullet would help force more gas back into the gas system? I don’t know much about the characteristics of black powder, so I’d be reluctant to make suggestions, but to me it looks like there’s just not enough gas pressure in the system. If you were getting enough gas pressure in the system to unlock the bolt, you’d just get a short cycle, so I don’t think the buffer spring is really an issue here. The issue here is that you need a bolt that takes less force to unlock, and that’s an inherent design characteristic. Could be the black just doesn’t have the energy to work an AR-15 gas system.