This may be just me, but I see a lot of misinformation in this thread. Using 60% bondo for this application is WAY too much. I used a 50/50 mix to finish my pieces and I would say even that is too thick for sloshing around inside a weapon. Also, as I mentioned, I use Rondo to finish all my armor pieces, and I have never used the tube of hardener that comes with bondo, I use the thin hardener that came with the resin. My pieces can hold up to field grade airsoft guns; I'm doing something right.
But aside from that, having completed two pep weapons, I would say that finishing a weapon by sloshing rondo around inside it is destined to fail. In order for it to by thin enough to slosh correctly, you're gonna end up using almost straight resin. And even if you do get the piece hardened using this method, it's gonna be very brittle and easily crushed. You'd be better off cutting the model in half lengthwise and finishing it properly using fiberglass mat (good luck getting it to line up again, though), or use the expanding foam method like I did.