....And just how do you think Big Brother will know about a "transfer" of a gun that was never in the system in the first place? Private sales of "unknown" firearms have taken place forever, and will continue to.
SRH
I think it will be as simple as grandfathering. There are many places now that require a 100% registration for lawful transfer. There'll be indisputable proof that a person was not of legal age to have accepted an unregistered transfer, if someone shows up at a range or gets spot checked on a hunt off in the future. Some places require proof of registered ownership to buy ammo specific to only that gun.
Don't some states now require registration of any firearm in the estate of someone who passes on? There may not be the ability to just say it's gone through private transfer. Some of ole gramps unregistered stuff may get flushed out, but even if gramps legally did a private sale, won't registered stuff come up flagged if it turns up possessed by someone else? I think the strategy is to let the last folks who think they have rights just die off, and the next generation will just assume all is good with the new normal.