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You do understand the issue is not with losing karma, but HOW MUCH, right?
I understand that you're not content with the amount of karma lost, but I'm taking this opportunity to point out that the best way to reduce the amount of karma lost is simply not shooting.
I'm aware that accidents do happen, and that sometimes you lose more karma than you should, but the situation that you're describing could easily be avoided.
If 5-10 people are in a giant group, and everyone is shooting everyone,
and there are explosive props nearby, you just shouldn't even fire your weapon at all. It's a horrible idea, and you have no case in saying that it's unfair that you lost karma because you blew up a barrel.
However, if someone tries to RDM you and you're quicker in killing them (Hence reducing the amount of karma THEY lose, and maximizing the amount YOU lose), that's a perfectly valid point.
Basic common sense from real life applies in TTT. Don't shoot when you're likely to miss. Even better yet, don't shoot.