You are wrong for that, it is not a bad idea.....it's an illegal and terrible idea.
SVX, You CAN NOT rig a lower, that was purchased as a rifle, to use a pistol upper.
You can buy a stripped lower reciever, AS NEW ONLY, and build your own pistol.....and have 15 pistol length lowers laying around and there are no legality issues. Even if the lower hasn't been built yet. That doesn't mean buy a second hand, stripped lower, if it was ever sold/classed as a rifle, it cannot be used as a pistol.
NEO, above is 100% correct - "B) If you don't have a pistol lower then you can't have any rifle lowers or complete rifles in your possession. If you do you can be charged with intent to manufacture an NFA weapon"
meaning - in your house, on the same property, in your possession.
When you purchase your lower from your FFL; I would strongly encourage you to have him list it on your form as "pistol" or just "stripped" lower. I have two of the spikes tactical "zombie killer" lowers that I'm building into pistols when I get home. one in 223, one in 45acp and a 300 blk.
It's not a bad deal, you just have to pay attention to detail. I've got a few suppressors at the house and waiting on a SBR/Suppressor & AOW tax stamps now. Mind your manners and don't be a wingnut and it'll never be an issue.
Cheers,
Breeze