I can't recall playing a roleplaying game where there was weapon penetration modelled. I mean, that when you shoot someone, bullet goes through him and does damage for an object or person behind him.
I am currently thinking about rule for ammo penetration for my roleplaying system YDIN. I have an idea technically how to do it, and without testing it yet, I think it will work fine. Actually, on paper (computer screen) and in my mind it sounds really good!
One thing I am wondering is, what weapons are capable to penetrate object. Is it about speed of bullet or about caliber? This I have to find out. I cannot imagine that .22 pistol can penetrate anything that well, but .50 handgun (cannon amirite?) would more easily. Or a robust rifle.
I have to find out what are the terms gun might penetrate an object or person. After I figure it out, I can write it down as rules.
Also, I have basic rules, character creation and advancement, skills etc. written down but haven't wrote yet combat rules. When I get this penetration thing done, I can start to write combat rules here also.
And that firearms penetration system... it will be the same with melee weapons. With same system you can penetrate your opponent with sword, axe or spear.
One thing in my mind developing my own system is, that you have these basics what are used for whole system. So you can easily write rules, and they will work as anything else. I don't like games, where skill checks, combat checks etc. are handled with different rules. Ofcourse there are some differences in rules in different situations, but I want the core of how things work to be similar with each other. And I want it to be that way, that if you want to mod the game, you can do it quite easily without heavily tearing apart whole system.