changing the gas tube is easy--no need to send it back to bushmaster
gas tube: simply get a pair of plyers and and yank the tube towards the reciever; h/w, if its like most used ARs, CLP the area where the tube goes into the FSB/gas block, let it soak as carbon buildup might have cloged the tube area
--if it gets tough to pull out, switch from a pair of pliers to a set of vice grips (line w/ tape if you are worried about scratching the tube
), lock it around the tube and pull towards the reciver
--if the vice grips dont work, then w/ the grips still locked onto the tube, get a mallet and smack the grips downwards toward the reciver until the tube gets loose
look to the area for FF tubes:
www.ar15.com/forums/topic.html?b=3&f=4&t=226782gas ring: a knife or sm screwdriver should pry/pop em off at the gaps; CLP clean the exposed areas, and slide the new ones on from the rear of the bolt