So besides the obvious ('68/'89 bills) what exactly are teh differences?

For instance, I know a post ban firearm can't have a bayonet lug, must contain x number of US made parts, etc... but why?

And in that case, why do people build 'em with import parts and the bayonet lug and stuff.

I was under the impression that the law just applied to full auto and/or select fire firearms... Apparently after some light reading, I was wrong.

I need some schooling.