on the underside of the barrel, about 3" from the recevier there should be a black pin that hold the barrel in place. If it is already loose then your job is easy, (pair of tweezers or something may get it out) but if it is stuck in there then i'm not sure how to get it out because it sits flush with the barrel.
for pics go to
http://www.arniesairsoft.co.uk/forum...owtopic=104695