Pretty much any help I could give you I learned from here:
http://www.zdspb.com/tech/mguide/sho...neumatics.html
Unless when you say its leaking from the barrel, you mean its reallllllly leaking like all the pressurized air is just blowing through it, i had this happening to me not long ago and it was the orings on my bolt guide, i replaced those and it stopped completely. Had me fooled, I thought it was something way worse.