It's a furry sticky mess that will poke you and make you in pain.
Thats why we do the inside of pieces, it isnt furry or very painful, but it can be if you do a messy job, and it has a wovem pattern which would be ugly for the outside of a AR or armopr piece, just dremel the piece down the middle after resining the outside, fiberglass the 2 halves, let them dry, then clamp them together and resin the outside again to reattach the two/.
People typically fiberglass the inside of armor and some of the weapons because you will lose the details when you cover a structure with resined cloth. It's like putting a sheet over your face, you lose a lot of visual information to the slopes of the fabric.
If you're making an AR out of pepakura, you're spot on. Just like AOB said, chop chop right down the middle. Aren't closed shapes fun hehe
OR OR OR OR!
all I can say is...if your willing to do this slightly suicidal attemp.
Resin the outside with 2-3 layers, let them dry, do one side first, then the other side. 3 layers done do the next step
drill a small hole in the back or the front of the gun, pour a cap full of resi nwith hardener into it, swish it around making sure the resin gets everywhere inside. let it sit drying for a month, resin without contact to much air or sun wont dry too fast, you could have a high powered lamp sitting over it too, to keep it hot and have some light shine in, that would make it dry in 1-2 days.