Here is a glitch for grey hammer and war head. It's an glitch i found i take all credit..
Crouch and walk into the corner once you feel your far enough in stand up and jump in.
This is what it should look like when you've successfully entered the tank.
This 100% works on grey hammer and i'm sure it works on warhead because there is the same tank there..
You can be damaged by nothing except i'm not sure about gas...