re: OFFICIAL Academy Reports/ Online Ammo Deals & Availability

I think you have your question backwards, or at least your concerns. Right now you have 2 legal ar-15 rifles, these consist of, more or less, a lower receiver with a stock, and a 16" overall length upper. So

2 rifle lowers with stocks
2 16" uppers

You buy a pistol lower with a cap or buffer tube only. Now,

2 rifle lowers with stocks
2 16" uppers
1 pistol marked lower

You are still good because: you can't attach a stock to the pistol lower, it is capped or has a buffer tube that won't accept one. A 16" upper on a pistol lower is still a pistol, a really long barreled pistol, but still a pistol

Then, you buy a 10" upper and put it on the pistol lower. Now,

2 rifle lowers with stocks
2 16" uppers
1 pistol marked lower
1 10" upper

Again you are still good because your number of "short" uppers is less than or equal to the number of pistol lowers you own. Just keep the 16" rifle uppers attached to the rifle lowers, and the 10" upper attached to the pistol unless you're cleaning or doing other maintenance, if you wanna be ultra safe.

Long and the short of it is don't have more sub 16" uppers than the number of pistol lowers you own, keep it 1:1 or less.