Muzzle Flash might not follow with all the lengthened and moved weapon parts.. Should not be an issue, since flash are very quick. (Or hope Diesel Model tool also support moving object/effect bone placement.. one can only hope :p)
well what do you know, 3 years later and the Diesel Model Tool have the capability to moves the muzzle flash and barrel_ext bones
the only issues is skin UV, Diesel Model Tool still cant perfectly re-export vanilla model skin UV, so the half of the barrel wont have pattern.