The problem is purely the fact that some guns are given perks that can't compete. If every shotgun had the same range and no range increasing perks they wouldn't be rampant. If fusions had little ammo, and if snipers had slower reload speed then the distribution would be even. For primary's it comes down to the first point I made. Re forging was great, but it failed because the base stats of hand cannons were just too good. But at the same time if every gun was the same then there'd be no variety. It's very hard to balance.