I think it could be fairly easy to add three distinguishable weapons without changing rifle too much if you guys are insistent on keeping it the way it is.
Shotgun could be a hitscan cone that pierces enemys(not blocks), meaning it could easily kill multiple people at short range, up to 15 blocks away assuming its a direct hit with no hidden parts or parts outside the cone(as it is currently, it will still be an insta kill close range even if only half of them are in the cone due to damage ramp up). In addition, because its a simple cone, damage would be consistent and no longer a luck factor, eliminating "shotgun sniping"(yeah right, that rarely happens anyways).
(a visual example of how far away 15 blocks is)
Outside of the 10-15 block insta kill range, the damage steadily declines, but is still pretty heavy for awhile, and at about 20-25 blocks, drops off to about 50 direct hit damage, eventually capping off at about 10 damage when its a "direct hit". The logic behind all this is to add stability to it in the form of the cone, but also offer it a distinguishable niche in the ability to kill multiple people in one shot, even if they're behind them. While some of you may groan, dont. The shotgun can currently do double, even possibly triple kills if luck favors you, this just makes it more common. In addition, they're sacrificing long range capability for this.
I like the idea of GM's Chaingun idea. While yes my idea draws a bit on the minigun from tf2, it works. Make it have the smg's current accuracy, reduce the recoil a bit (it is heavier and bigger mind you) from the smg's recoill (it should be near nonexistent when crouching), reduce the firing speed slightly and buff the damage, im thinking to rifle damage, or slightly greater. I know what you're thinking, clearly op. This is balanced it can only fire after revving up for 3 seconds, during which it's movement speed is reduced heavily(im thinking to a mere 25% of what it was) and also generating a loud noise that can be heard up to 25 blocks away. In addition, it has no iron sight, instead RMB will simply keep it revved in anticipation of an enemy. The idea is to offer a high damage, slightly high fire rate weapon that cannot be used to run and gun with, as its far too slow. The bullets will be somewhat accurate, but also contain the ability to one hit, allowing it to lock down areas for a bit(before i forget to mention, Block damage by guns will be severely nerfed, taking like, 10 shots to destroy a block with the chaingun).
Feel free to flame me and tar and feather me