i'm guessing it might have something to do with your UVs not being layed out yet

i just took a look at your mb file and looked at the uvs (you can see it in Edit UVs > UV texture Editor)
the trigger and all cylindrical objects seem fine (albeit oversized) but the rest of the gun is messy

you may or may not get somewhere if you lay out the uvs, i've not heard of the program you're using