I doubt we'll find anyone who's been hit in the chest by a 20mm HE round and survived to tell us what really happened.
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I mean... the same could be said for a .50 cal, yet that can still happen in the game.
My whole point in that very long reply was this:
This is a game.
A low damage roll from any weapon should be interpreted by the GM as a near hit or grazing shot.
How else would you explain someone surviving a shot to the chest from a hunting rifle, doing only 3 damage (which isn't even enough to cause a crit)?
So, like I said (in very many words), a 1 damage hit from a 25mm HE shell while wearing a flak jacket would have to be explained away as a "fluke shot that glanced off you right before exploding" or something like that.
Otherwize, you'll have to start making this game a WHOLE lot deadlier, since most people don't survive getting shot by any sort of rifle bullet without modern medical treatment. (And I'm not talking about the personal medkits that most PC's get)
Is it possible to survive a direct hit from a 25mm HE shell in this game?
Yes.
If you're very very lucky, and well protected, and they rolled a really really crappy roll out of their (Potentially) 8 die roll, and both hit locations impacted only the area protected by your flak/helmet.
Most likely, though, you're SOOL if you're fired at by a 25mm chaingun using HE.
PS. you can also argue that it's unrealistic that a .50 cal hitting someone square in the chest while wearing flak armour only does 3 damage if you only roll 1 hit... and if you roll a 1 on your crit roll, you only get a snapped collarbone. You don't even get knocked down by the impact!
See how this is starting to sound ridiculous?
Yes, you can survive many things in this game that you probably shouldn't.
But the alternative is to make almost every single shot lethal.
And that wouldn't be too fun.