I would not recommend to come up with weak spots yourselves (unless you see an interaction based on previous description, there is nothing wrong about adding descriptions/content to the game as long as it does not affect the actual outcome

). In this case I'd recommend to first request further description of the barrel. (which can be split in either just more detailed description, or a request for full-turn close up investigation).
In this case I interpret your description as "fire at any visible weak spot on the barrel" and there are no indications that they are filled with kerosene. These are standard-issue nuclear waste barrels that explode due to radon build up inside, and majority of the damage comes due to thermally hot contents, rather than ignition...
In any case, at 2 shots that you fired, you got 84, 29 (btw, 2 shots take 50 TU, not 40, but Ill let it pass this time)
:
Pvt. Mori glanced at the barrel, considering what would make it die faster, he fired off two shots, one of them missing wildly, while the other penetrated the barrel just near the floor, a very slow goop of green mass started to emerge from the small hole. (barrel took 20 damage, resulting in 130 health remaining, this info would not be posted in the full game)