Editing "TRAINING.DAT"

Well if you extract WORLDS.rez using RezXtract then you get .dat files for a bunch of maps. One of them is training.dat and I am trying to change the default M24 they give you into a minigun or pretty much any other weapon. I have seen a guy change it into an airstrike-LGB so it must be possible. When you search for "M24", you get this:
and I have tried changing both of these into M136_AT4 (ss rocket launcher) and I even checked in GUNS_M_HH (models of guns when they're on the floor) to get the exact filename and then injected training.dat into worlds.rez but whenever I try training mode doesn't load Here is a youtube video showing the finished product with an LGB: I am a 99.5% noob coder, as the only editing I've ever done to rez files is changing training mode into different maps. I know that this is my first post, so nobody on mpgh knows me but I'm hoping that you guys can help. All helpful answers get any rep/thanks I can give and maybe some sort of in-game gift. For some reason it won't let me type ltc but that's what the blocked out part in the second part says.