Bullets are ubiquitous, unserialized, and people already own them. I would have to think that applying an excise tax on already owned property would be a major violation of ex-post facto implementation of a law. Prior property would have to be grandfathered to avoid constitutional issues.
And let's be realistic... a $5000 tax per bullet is comical posturing. I could see MAYBE $10/bullet... $100 if the stars were aligned just right. And here's the thing, loading an AR mag, even @ $100/bullet, would only cost $3000. Do you really $3000 is a deterrent for some suicidal douchbag about to shoot up an elementary school?
Lastly, none of this even addresses the fact that many recreational shooters MAKE their own ammo because it's cheaper and usually gives better results than store-bought ammo. They reuse the ejected brass dozens of times, bullets can be formed from melted lead weights, jackets can be swaged from scrap copper/brass, and primers are literally about $0.02/each. A hundred or more rounds of better-than-factory ammo can be made at home more cheap than factory ammo in just an hour or two. A black market for "unregistered/untaxed" ammo wont even have to exist... people and criminals will just make their own. 
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