I also feel like if someone wants to spend an hour dragging barrels around to make a massive explosion and kill something they probably shouldn't have they should be allowed to do that. This is a very pen and paper thing to do. The only thing I'd change is maybe let NPCs notice you're stacking barrels around them and react to that in some way. Or have someone get mad at you for running off with a barrel full of booze. I know I'd be pretty miffed if I saw someone do that!
Or they could address the fact that 1 cubic meter of water weights a ton (not hyperbole, I mean a literal TON) and that not even an ogre would be able to launch full barrels approaching that volume around up to meters away as if they were stones, let alone humans and co.
When "sticking to a degree of realism and common sense" and "fixing a potential exploit" can be solved in one shot. A proper matter of killing two birds with a stone.
