I've used West Systems epoxy on boats, too. And, it's good stuff. No reason it wouldn't be great for hidden areas on a gun.

BTW, I repaired a crack in the wrist on a buttstock given to me for my Perazzi comp gun. I did that with the Satellite City cyanoacrylate about 13-14 years ago. It's been shot hard since then and it's never showed any signs of re-cracking. The good thing about using this type glue on external cracks is that it's so easy to hide the repair. JMO, YMMV.

May God bless America and those who defend her.