You need a prog called O2
And DePBO
Extract the .pbo file, and open the model file in O2
Cut the muzle flash, save, paste it (It will return to right place)
Save it again
Now use Make-PBO to to make the folder into a PBO
That should do it
It's nothing to do with the CPP file, sadly

Not the easiest if you've never used O2
Only other option, is, I'm afraid, live with it

Or complain to the makers

Gd luck
- Ben