My proposal says to simply changed '1898' with '1928'.
Don't get me wrong in this, I really like your idea and I think it is a step in the right direction, even if it does not go nearly far enough.
Your change would mean that a Mauser sporting rifle manufactured in 1932 could be aquired with no background checks but an otherwise identical Charles Daly sporting mauser would have to go through a 4473. Or an 03-A3 could be bought off the shelf but a Savage in 30-06 couldn't.
This will not fly because there is no difference in form or function between the 03-A3 and the Savage. I can drill and tap the 03-A3 (the horror of it) and have a rifle every bit as good as its' modern equivilent with no paperwork.
So in that sense the change makes no sense. The logical conclusion would just be to rid ourselves of the '68 GCA entirely (which would be a good thing). This change would open the door for such a push to gut the '68 GCA on the basis of the form and function argument so it won't happen.
The only modern rifles that currently count as antiques are the early small ring mausers which are not known for their immense stength, unlike the later large ring mauser, which falls after the 1898 date. Ever wonder why the date on the law is 1898 and the large ring Mauser came out in 1898. This is not a surprising coincidence.
All the other catogories of difference are based on actual function. Muzzle loaders using black powder are not firearms regardless of when manufactured, so date is irrelevant.
Also most senators are not gun literate, particularly old gun literate. I collect C&R rifles and the oldest rifle I have is from 1920 something.
It's accurate and I can happily shoot modern 8mm hunting ammo through it. And yet there are people I meet at the range who ask "do you actually shoot that old thing." Errm, yes, and it can still shoot better than I can. I strongly suspect your senator thinks a 75 year old firearm is fragile, rare and on the verge of not being usable.