Show me you've never studied military history without saying you've never studied military history.
Normally I wouldn't bother with such a clearly off-topic response, but that was just so bad I had to.
Amphibious operations are notoriously difficult and get progressively more difficult the further from your secure resupply bases they are. Add on a continental opponent with a massive population and secure industrial base (the United States has been that for close to two centuries now), and the amphibious lift requirements, as well as the naval firepower needed to escort that lift, become astronomical - we're talking the need to transport literally millions of soldiers and millions of tons of equipment and supplies. The logistical requirements would dwarf the entire modern global supping industry.
You could not only eliminate every civilian firearm, but every civilian, police, and military firearm in the country and not reduce American security against foreign invasion by a sliver. The oceans alone are a nigh-insurmountable barrier, and are protected by naval and air power that are the equal of the entire rest of the world combined.