Yep too many variables. Hes shooting .45 ACP so pressure isnt likely a cause. Due to the low pressure and low velocity of the .45 ACP that all but rules out partial obstruction like too much oil. In my mind the only real possibility here is a stuck bullet.. Perhaps a bridged powder dump from the measure and the round that stuck only got a half grain of titegroup or so. Which means there could be a round in the rest of the loaded ammo with a near double charge..
Or best case scenario.. A round simply didnt get charged with powder and the primer just pushed the bullet an inch or so down the barrel where the next round fired normally collided with the stuck bullet and both went on to the target..