I think TM 5-7 is known to break over time because of propane use. The breakage occurs in some major part and not just cracking in the slide/barrel. I have a TM 18c and so far propane seems to be working well. I have a stronger recoil spring in there but I'm certain that eventually the plastic slide with give way. Will upgrade to a metal one when that happens.
H&K G36c (KWA) / Glock 18c (TM) / Kimber LAPD SWAT Custom (TM) / Mossberg M500 (ACM)