Nice. Can you make the feed out of tubing and just toss the ACP mags? They just add bulk in my opinion and unless you are planning on swapping them during the game you don't need more than a normal spring fed tube attached to the body. It could (probably should) be removeable methinks, but you could make it a one piece job. 12gs at that pressure should get you 30 shots I think, depending in how the valve is.