Hiya, I know that this has never been suggested before so, The Elephant Gun.
During the early days of WW1 when tanks started to be used for the first time, average rifles could not penetrate these armours, the one thing they did have in the 1910's, was a gun so powerful it took down Elephants
(The Wikipedia article on its use in warfare)
http://en.wikipedia.org/wiki/Elephant_gun#Use_in_war
How it could be adapted to gameplay is it would be the anti block gun penetrating upto 5 blocks in one shot and achieving insta-kills on anypart of the body (exception being the limbs). To balance this monster out, it is a double barrel rifle requiring constant reloads.
How do we all think about this?