I think guns and rifles are rather a question of providing players with different options regarding fire rate, fire rounds and destructiveness, combined with the level of accuracy of a unit. This will make the basic physical constraints, time and distance, important factors in employing and developing tactics.
A heavy gun which is not highly destructive but has high fire-rates and fire-rounds, is good for close combat, as even with low accuracy, chances are high that you can hit the enemy several times without running out of ammo. But its a waste when a low accuracy unit fires a whole clip on an alien which is on quite a distance. Typical example: Machine Gun.
To the opposite, a heavy gun with low fire-rate and low fire rounds, but high destructiveness, would be a good way to kill enemies before they get too close, especially when your unit is one with high accuracy. Noone wants his ammo to run out when the alien stands in front of us. Typical example: Sniper Gun.
Variations could create interesting combinations of fire-rate, fire rounds, destructivenesz, maybe even featuring special ammo types. Some could use these features in extrelemely polarized ways. Like they could be highly destructive, but fire at very low rates, very often not more than two or three rounds with a single clip. An example for extremely polarized stats is the rocket launcher. Just talking about rocket launchers, you could also have a class of intelligent guns, with guided ammo. These would eliminate the problem of accuracy. But they would be either expensive, rare, low ranged or a burden on the units mobility.
Then there could be a class of special purpose weapons. Purpose is more important than the fire rates etc here, as the name already suggests.
There could be made a distinction between "aggression" weapons and "defense" or "just in case" weapons. I rather would attack an alien with a heavy gun than a light gun. But even with a heavy gun in hands, I would like to have a light gun in my inventory, just in case... you know.
I think there must be a class of general purpose weapons, able to kill all sort of aliens. In the long term, when accuracy has increased amongst all units, the significance of the gun type will parish, since time and distance will not be as important as they were when accuracy rates were low.
