Boy, this one is a big Big BIG can of worms.
The best quick answer that can be given is that it depends upon where the gun was manufactured, or at least, where a majority of the main parts of the weapon were manufactured, AND, where was the magazine manufactured. (Leaving out, of course, the worry for civilians that the magazine itself be a legal pre-ban magazine.)
If the weapon and the magazine were made entirely within the united states, and the magazine is a legal pre-ban magazine, then there would likely be no legal problem with this set-up. (I suppose I could construct a hypothetical involving an "assault-pistol" that should not have removable magazines or that might be pushed over the weight limit by a 30 rounder
, but for now just assume an AR-15 (post ban) or clone and a pre-1994 magazine then you would have no problems.)
This all gets A LOT more complicated if the weapon is a post-ban import built of imported parts on an american receiver. Then all bets are off and each should be looked at on a case by case, weapon by weapon basis.
[This message has been edited by Dizzipator (edited June 07, 2000).]