Okay, I'll throw in my two cents here. In order to fiberglass your piece, the fiberglass itself will need resin coated onto it to actually cause it to harden and strengthen. And you do this by mixing your resin according to the instructions on your can, it varies a little from brand to brand. It comes with the actual resin, and a small tube of hardener that should be enough for the whole can. Normally, it is also recommended to apply a layer of resin to the outside of your piece and let it completely dry before you fiberglass the inside. It helps to keep your piece from warping from the weight of the wet fiberglass. I also recommend taking a bit of glue, and cutting strips of fiberglass and glueing them in place on the inside of your piece making sure to cover the whole thing, overlapping your pieces a little bit. Then you'll mix up a fairly small batch of resin (it dries quickly, normally in about 10 minutes or so. You will then want to take a paint brush, (I'd recommend getting a few really cheap ones, I like to use 1 1/2 inch or 2 inch brushes) then with that brush, you'll want to dab on a pretty healthy amount of resin, "IF" you glued the fiberglass in place, you can brush it on as well, but I find that it soaks through better when you dab it, You wanna make sure it soaks through pretty good for the best effect, but at the same time you don't want you're whole model to get all soggy. So don't get too much in one spot, if its pooling up, spread it out with your brush.
After you get a complete layer, you should let it dry between coats, but I know some people continue layering on 2 to 3 coats right away. I'm not sure which way is better, but I think its easier to do one then let it dry in a warm place for a couple hours. If it still feels sticky to the touch but seems mostly hard, use some baby powder to help get rid of the stickyness. I'd say depending on the piece, you'll want at least 2 but possibly 3 or 4 good layers of fiberglass for strength. Just depends on the piece and amount of pressure it will be under, like the butt plate, if you sit down much you're gonna want it real strong.
I hope this helps you out some, as for the Assault Rifle, unfortunately I don't know much about fiberglassing them, but I've heard people cut them in half to do the inside, you really do keep a lot more detail if you put the fiberglass on the inside.
Anything else I can help you with?