In CoD the guns don't get better as you go up the tiers, unless you consider the crossbow or ballistic knife the best guns in the game(they're the last tiers in Gun Game on Black Ops). In fact, I believe as you get higher in the tiers they're supposed to get more challenging(it wouldn't make sense to reward players with better guns if they're already doing good... then there would be no way for the other players to compete). In MW3 the guns don't really get worse at higher tiers, but they don't get better either.

Not to mention saying "guns get better" when applying it to CoD makes little sense as there's no suppose to be guns that are way better than others as their goal is to keep balance and offer variety(not saying they DO). They most certainly did not design Gun Game to reward players with better weapons as they get kills as they didn't design the guns on a tier like that(with some guns sucking and other guns being way better).

You can take a look at the tiers here: http://callofduty.wikia.com/wiki/Gun_Game