By tracking ammo and accessories the ATF will be able to identify by type and quantity of ammo where the guns purchased on the secondary market and guns gifted to individuals ended up at.

If you shoot with any kind of regularity, they will have their back door universal quasi registration / accounting for door to door confiscation. They will know what you have and how often you shoot.

This of course will create a gray / black shadow market for ammo at a premium price.