The gun banners want to go after the process rather than the outcome. The choose to ban items based upon potential. A perception of lethality based upon what is seen in a sensational action movie becomes the common wisdom used for determining what is too dangerous for a private citizen to own.

The gun-banner looks at a fragmentation grenade and decides it is too dangerous because it could kill or injure a lot of people, especially based on the movies where fragmentation grenades blow cars into the sky with giant fireballs. It is the potential for destruction that causes it to be banned.

Applying the same deadly force standards to fragmentation grenades as firearms it would be a case of ensuring that all of the people injured were deadly force threats at the time the grenade was deployed. It is the actions and state of mind of the defender coupled with the actions of the assailants that determine if lethal force is justified. The mechanism is not relevant. As soon as a person is injured or killed that was not an eminent, lethal threat then it is a crime.

As a practical matter there isn't much of a defensive purpose for fragmentation grenades in the hands of private citizens. The cases where their use would be appropriate are extremely rare and this limited utility is outweighed by the practical difficulties of safe storage, training, and expense.

The concern over potential is what leads to the restrictions and bans by caliber, capacity, and ergonomics. The bullet doesn't know if the gun that fired it had a pistol grip, bayonet lug, or detachable magazine, and neither does the emergency room physician or the medical examiner.

Ultimately potential lies with the shooter, and not the firearm. The sane, moral, law abiding citizen doesn't have the potential to shoot up a school or lob frags into a shopping mall. By the same reasoning the insane, immoral, criminal will find another mechanism for inflicting destruction: The Green River Killer strangled, suffocated, or stabbed more than 40 people. No amount of mechanism centric regulation would have prevented his crimes.