I have had a PA 10 lower for some time and recently my wife bought a matching upper from psa. I have had enough spare parts from upgrades on my other m5 recievers to build it out. All I can say is WOW, the amount of proprietary parts is nuts. Take down pins as well as bolt catch. Had to take a carbide burring tool to the gas tube entrance to get the tube to go in on the upper receiver. Went with a 16 inch ba modern profile barrel with a sa adj gas block for my suppressor. I hope this thing shoots this weekend, should be interesting.