ds1973, I've seen some containers at gun shows that would hold an AR15 if the stock is removed. They're military surplus containers for, I believe, howitzer ammo. Not having been in the military, much less the artillery units, I can't tell you exactly. But they're watertight, have screw on lids with o-ring seals, and would do the trick. The guy selling 'em charges about $30 each.
But it's a lot more fun to do some work for pro-gun candidates, and then go shooting, than it is to think about the guns you've had to bury somewhere.
Dick