very little, unless you get caught with a banned firearm after the expiration date.
even then, there'll probably be amnesty if you turn it in voluntarily afterward - you just won't get paid.
but, let's be real. in spite of the rhetoric coming from both sides, there will probably be a compromise in any bill before it passes. "grandfathering" is one option. licensing box-magazine fed semi-autos the same way we currently license fully-automatic machine guns is another.
please keep in mind the human nature that one extreme invariably draws out it's own opposite.
the NRA says "no regulations or universal background checks, PERIOD." that's pretty extreme.
the gun-control mantra is "we'll confiscate every gun."... and there's the opposite extreme.
there is a vast chasm between the two sides where there is an opportunity to do something to reduce the number of battlefield weaponry in the wrong hands.
(and don't try to tell me an AR is not a battlefield weapon. the rifle itself and the ammunition was specifically designed to be people killers. i'm a machinist. i can convert any civilian gas-operated, box-magazine semi-auto to fire full automatic in less than 15 minutes. that includes disassembling and re-assembling the lockwork.)
you want a WWII-era M1 Garand? i doubt you'd find many democrats who'd object to that.
1) it's a high-powered cartridge. .30-06 Springfield to be exact. it's got a helluva kick, is loaded through the bolt rather than a high-capacity box magazine, and only holds 8 shots. not exactly the sort of thing to have if you're intent on causing a mass-casualty event. it can be used that way, but AR's are much easier to control and much more efficient at killing large numbers of people as quickly as possible.
2) while the .30 caliber projectile is deadly enough, it only punches a half-inch hole through a human body. the .223 Remington cartridge that most assault rifles are chambered for is much more lethal. it was designed to be unstable. upon striking flesh, the bullet begins tumbling end-over-end, resulting in horrific wound channels and quick death by exsanguination. (i learned about this when i was being trained as a medic in the US Army.)
... and while i have a great deal of sympathy for the gun-control crowd - my wife having lost a God-daughter at Sandy Hook - there's little chance anybody is going to "come for" your trusty Browning Auto 5, Remington 700, or even your S&W revolver or pistol. too many of us democrats have one or all of those.