He has been vocal and he says point blank. AR-15 either sell to the feds in a Buyback or get your paperwork done for an NFA.
From his page.
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It's not clear weather you'd need to register each and every "high capacity" magazine as an NFA item but if so 5 magazines would be $1000 in NFA fees and who the hell know however long it takes for the paperwork to clear.
He also said he'd ban online sales of firearms, ammo, and even parts.
" End the online sale of firearms and ammunitions. Biden will enact legislation to prohibit all online sales of firearms, ammunition, kits, and gun parts. "
Joe Biden doesn't know shit about firearms and is no friend to the 2nd Amendment.
"[if] you want to keep someone away from your house, just fire the shotgun through the door." -Joe Biden Feb 2013
"So instead of anybody coming at you and the first thing you do is shoot to kill, you shoot them in the leg." - Joe Biden Oct 2020