If I fabricated a buttstock with an integrated pistolgrip connected from the bottom of the grip to the tail-end of the buttstock, would the pistolgrip not be considered a pistolgrip? If not, I believe I could mount a pre-ban upper or a 20 rnd magazine on it. One or the other, not both.

are you talking about making a thumbhole stock? because i read on the atf site that those are considered to have pistol grips also, unfortunately.

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I'm assuming you're talking about AR-15's.

As if it makes any sense at all, for purposes of the '94 Assault Weapons Ban thumb hole stocks are considered pistol grips. But, for the earlier import rifle ban thumb hole stocks are not considered pistol grips.

So, in answer to your question, no you can't replace your stock and pistol grip for a thumbhole stock and put a preban upper on your post ban rifle.

As far as a 20 round mag, if you are talking about post ban AR's then its ok to use pre ban 20 round mags in them.