The cut of the barrel is all about the hopup application hole, and the things around it. A bull barrel is simply a barrel with a 10 mm outer diameter, which reduces vibrations.
PDI has some rather unusual barrel lengths, check out www.x-fire.org, and they should be able to help you out. If not, go to Edgi, he can make custom barrels for you, if you need one, but he may have one in stock, just for you
VSR barrels are not compatible OTB with AEG barrels. The PSG-1 uses an AEG cut, as far as I know, so no, it would not be compatible.
PDI has some rather unusual barrel lengths, check out www.x-fire.org, and they should be able to help you out. If not, go to Edgi, he can make custom barrels for you, if you need one, but he may have one in stock, just for you
VSR barrels are not compatible OTB with AEG barrels. The PSG-1 uses an AEG cut, as far as I know, so no, it would not be compatible.