Were the government to limit the amount of ammunition made and sold in the United States, there would still be an awful lot available. James Holmes bought 6,000 rounds online before his shooting spree in Aurora, Colorado. Bullets are so easy to come by that it's clear that huge stockpiles exist throughout the country. But unlike guns, bullets are single use. You fire a bullet, you expend its propellant. While attempts to remove guns from the streets would either be incalculably slow or require heavy-handed, dangerous government action, curbing the ability to buy ammunition would mean a natural diminishment of the arsenal that remains. Every time a bullet is fired, that bullet is lost forever.
This reminds me of Roy Amara's remark that we tend to overestimate the short-term impact of a technology, but underestimate it in the long-term. What's the score on small-caliber ammunition that's good at incapacitating people but not much good at killing them? Taser was working on "nonlethal" shotgun shells, but it all seems a long way off from becoming a meaningful (or regulable) decision to consumers.