Ohio Congressman Bill Johnson made this argument the other day against limiting high-capacity magazines (at about 1:10):
Well, where does the limit stop? When you stop that, then do you start taking away handguns, do you start taking away shotguns, do you start taking away multi-round hunting rifles? You know, where does it stop?
Johnson is aptly demonstrating what is known in rhetoric class as the fallacy of the slippery slope (aka the Fallacy of the Beard, the Camel's Nose, the Boiling Frog, etc., etc.). The answer to his question, under Senator Dianne Feinstein's bill, would be 10 rounds for all magazines. The Feinstein bill also bans the sale, transfer, manufacture and importing of semi-automatic shotguns with a fixed magazine that can take more than five rounds.
(Video and interview from Ohio Capital Blog)