[tobnpr]

Certainly no expert in NFA here.
But, ATF is clear- being "in the business" of buying/selling firearms for profit- NFA or not- requires a firearms license. Sounds like a slippery slope to me- it would be interesting to hear what the attorney you retain, has to say about the proposed venture.

To my knowledge, it is irrelevant how long a firearm is "held" for investment.
By your own admission, the sole purpose of the corp- is to buy, and sell NFA weapons for profit. IMO, you would need a dealers license at minimum even for transferable NFA.