This happened to me, once... A painful but funny experience...
what you should need is a Hammer, FlatHead screwDriver, and some pliers,
the first thing I did was get the Barrel between my legs to hold it, then took the screwdriver and held it at an angle and started tapping with the hammer to try and loosen it up, and that didn't work to well it only broke some pieces out but not all of it, so then I got the pliers and pressed as hard as I could onto the side on the nipple and twisting at the same time, it then started to unscrew and then it came out, I'm not good at explaining things so if this doesn't help maybe someone else can
How this happened to me, Well what I can say is if your gonna shoot a water at 100Psi "Hold on Tight!"
My cannon is 5'4" long, the back is a 2" diameter chamber and the front barrel is also 2" in diameter, I use sch80 for the nipples, anyway I filled up the barrel with some water with the cannon at 100psi, being a stupid ass and not thinking I held it like a bazooka and flipped the Manual release valve, it then rocketed back out on my hands, went at an angle and the barrel hit me across the neck and chin busting the nipple into three pieces, I was in pain but laughing,
if I build another(I've had plans made about a month or more ago in photoshop) It's gonna be a 4" diameter chamber with a 3" diameter barrel, and Metal nipples and so on...