A standard 115-year old pump-action shotgun w/ 0-0 buck would have created every bit the carnage as the Bushmaster.
At this point, only the Illinois proposal might have an effect
and at that point, use of a samurai sword over a 30-second timeframe would yield the same damage -- only far, far more horrifically so.
You just have to keep walking their arguments back on them, one weapon at a time, as they splutter in their cocktails.
Pretty soon they run out of ideas and revert to pounding the table.