Even though we commonly use "pre-ban" and "post-ban" as short-hand to describe uppers, that's really a false description. Even the terms "pre-ban" and "post-ban" aren't quite correct.
Rifles that were in "assault weapon" configuration on or before Sept 13, 1994, and were legally possessed at that time, are grandfathered under the law (exempt from the law that would have otherwise banned them). We call these rifles "pre-ban" for short. Any rifle that doesn't qualify to be grandfathered cannot legally be made into "assault weapon" configuration. We call these "post-ban."
An upper by itself isn't anything. However, we assume that an AR upper will be put on a lower that accepts detachable magazines and has at least one of the "evil items" on the list: a pistol grip. The lower could also have a collapsable stock, which would make the lower an AW under the law even without an upper attached. Anyway, because the assumption of the lower is made, we generally refer to uppers that have any of the other "evil items" as being in "pre-ban" configuration. The other evil items (the ones that could be on an upper) are:
- flash suppressor or threaded barrel that could accept a flash suppressor,
- grenade launcher (any 22mm muzzle device qualifies, in addition to M203 type launchers)
- bayonet lug
A "post-ban" rifle is defined as a semi-automatic rifle that accepts detachable mags and has either zero or one of the 5 evil items. On most post-ban ARs, the one allowable item is the pistol grip. An upper with none of the evil features listed above may be legally mated with a post-ban lower.
-Troy