Simply ........ if there are 2 guns in he game, and only 2 guns, but those guns dont have identical stats. people will complain they arent balanced.
The problem is that people want the gun they have in thier hand to be equal to every other weapon in every situation. If that were so, sure the weapons would be balanced ..... but the game would be boring as heck.
They dont give you a choice of dozens of weapons so that the weapons can all be identical.
Game balance is the key .... not weapon balance. Use a weapon that suits your tactics and situation and you can find balance.