That argument was already used to ban the full auto versions of those weapons and now you want to ban the semi-auto versions as well... Seems pretty slippery to me. More importantly, that argument does not explain why you believe that I, or any other law abiding gun owner, should be punished for the actions of criminals who ignored the law.
A ban would only ensure that you keep such weapons out of the hands of lawful citizens, like myself, while criminals could still obtain them on the black market. Since criminals are perfectly capable of illegally importing cocaine and heroin despite their bans and an expensive "war" on drugs, what makes you think the bans you propose will do anything to keep such weapons out of the hands of people who would use them "to efficiently kill as many people as possible in as short a time as possible"?