An assault weapon is a rifle that looks scary; they're functionally not very different from a standard rifle. A grenade launcher is already heavily restricted, and civilians can't buy actual grenade ammo for them. As for bayonet mounts, I have yet to hear of a single crime committed with a bayonet, probably because it's far more effective to use the knife by itself. Stock and grip changes, as well as muzzle brakes, provide accuracy increases, but if reversing that was the goal they'd ban optics. A barrel shroud prevents you from burning yourself. A flash hider sounds scary, but doesn't do all that much for anyone who isn't hiding from a trained sniper spotter. Silencers are already heavily restricted.
If you meant an assault rifle, which is actually capable of firing more than one round per trigger pull, then they're already crazy expensive and highly restricted. They're also not used in US crimes due to their overwhelming expense and the difficulty of obtaining one. The few crimes that have occurred since the Valentine's Day Massacre in the 20s have mostly been law enforcement officers who have access to the assault rifles in their local police armory.
The problem with gun registries is not their usefulness to law enforcement when times are good. It's their usefulness to tyrants when things are bad. Unfortunately, the US has a long history of abusing data that's been collected for other uses. Other countries have even worse records when it's come to using firearm databases.