I'll be honest. Today's post is inspired by a video game (Planetside 2 to be specific) and not a table top game. Still, the question comes down to a matter of game balance and feel that isn't specific to just videogames and is actually applicable to all games. The question being should everyone be able to kill a tank? Now, tank could be anything. It could be a dragon, or a space ship, or a super powered juggernaut. The point is the same. Should everyone be able to kill it? Or should you need certain items, equipment, or abilities to have a chance? Let's look at some of the thoughts that go into answering this question.
Roles and Niches
Leaving aside arguments of realism (we'll touch on that later) one of the first things you may want to look at is the concept of a role or a niche. Whether you have classes or not people in your game are going to subscribe to certain roles and niches. This is how we get to make our characters feel unique. There is an area that they generally handle. The mage casts spells. The fighter fights people. The thief sneaks around and steals stuff. Even if we didn't have classes you'd still end up with people focusing on magic, fighting,or sneaking. Sure, there'd be more overlap, but people would still define roles for themselves.
So, why is this important?
Well, because tank killing is a job. Now, if everyone can do the job you end up with two problems. One, who is the best at tank killing? Two, what is the point in a tank killing class if everyone can do it?
On the other hand, if only specific people - or specifically equipped people - can kill tanks yo have two problems. One, why can only that person do it? Two, what happens when that person is not around?
Deciding which of these two questions you want to answer will make the decision for you. Do you want to worry about balancing just how fast different people can kill tanks and worry about roles and niches in other ways? Or do you want someone to have the job of big damage to big things, and then have to deal with the problems that arise when there is no big damage to big things person around when a big thing shows up?
Realism vs. Heroics
Now the real argument a lot of people bring up is whether or not it is realistic for just anyone to be able to kill a tank. Should that guy with just a hand gun and a grenade be able to kill a tank? That's not very realistic now is it. How unfun would that be for a player in the tank too, if some guy with a hand gun and a grenade could take him out. This might work as an answer for a computer game with limited options, but what about an RPG? One of the strengths of an RPG is that the mind's ability to come up with creative solutions is not hampered. So when a PC manages to sneak up onto the tank and tells the GM "I open the hatch, throw my grenade in, and shoot anyone who comes out" the GM can roll with it. On a fun note, that is a way that - in reality - a guy with a gun and a grenade could take out a tank. Provided they could open the hatch anyhow.
Unfortunately, it isn't as easy as saying "a person could do it so yes, anyone can do it" because with that you're mistaking heroics for reality. Sure, heroics can happen in reality, but it isn't a normal case. That guy with the grenade and gun getting on top of the tank, opening the hatch, and getting the grenade in is a pretty extreme case. Does that mean the rules should allow for it though? Without the application of creativity should it be possible? Should that hand gun have a chance, incredibly small as it is, to penetrate the tank's armor?
In reality the answer is no. Certain caliber weapons simply can't do much against certain levels of armor. Other kinds can. This is part of the reason for heavier calibers and different sized weapons. Would we have even developed armor piercing rounds if regular bullets could do it? Maybe, but I doubt they'd look like they do right now.
It All Comes Down To Feel
The point I'm trying to make here is that it is going to come down to the feel you want in your game. If you want a person to have the role of tank killer, than you should curtail other people from being good at it. Why? Well, to protect the niche and keep it a role. If you want the game to work like Halo where enough assault rifle rounds can kill a tank, then go for it. If you want the tank to feel safe wading into a swarm of infantry then go for that instead. The question is always going to come down to the feel and the themes of the game you are making. Just, when you make that decision, make it for the right reasons. Know why you made the choice and what your answers were to the small questions that make up the big one.
Who knows. Maybe in your gritty sci-fi game world you end up with normal rounds being able to kill a tank, or heavy weapons to not be as restrictive as they otherwise could be, just because you don't want the players to be crippled because two people wanted to play scouts and no one wanted to carry the heavy toys.