"West System" is an epoxy resin used in the marine industry alot. They offer many different fillers to thicken it. They also offer a couple different hardeners for different cure times.
I mess with boats alot and also use it frequently in my building business.
I used the West System to build a drift boat several years ago "incapsulated plywood", it worked well for the boat. For rifles, I wouldn't use it unless I had to. Do yourself a favor and get you some Devcon steelbed putty and use the best.