9mm AR build.

[AdamP123]

The rule as I understand it, is you can turn a rifle into a pistol, but not a pistol into a rifle. Confusing I know.

You have it backwards.

But of course the [strike]pistol[strike] RIFLE would have to have a barrel over 16" or it would be a sbr.

Fixed it for you.

I think I may have found the answer to my question by the last comment before mine but here goes. I am working on building an AR9 rifle. I will purchase a 9mm upper but I was looking into using my rock river lower. I would have to add a mag block for glock mags and I am finding either a 9mm buffer spacer or a new buffer it self. My main question is could
I purchase a complete pistol lower and then change the tube to a carbine tube and add a heavier buffer for a dedicated lower would it work. Changing a pistol lower to a rifle is or
isn't possible?

You can have a pistol lower setup in pistol configuration. Then, if you wish, you can throw a rifle upper on it and then put a stock on the lower, thus creating a rifle from a pistol. At that point, it is a rifle. However, do not put your pistol upper on it while the stock is on the lower because you would have just created a SBR (because the barrel will be less than 16" with a rifle stock on the lower) and, without a tax stamp, that is a no-no. It seems rather odd and nonsensical, but there is a lot about the NFA that is outdated and makes little to no sense. You can turn a pistol into a rifle, back to a pistol, back to a rifle, so on and so forth, but you cannot do that with a rifle. All of this gets somewhat sketchy, especially with some of the newer braces that attach to a standard carbine buffer.

With as well as some of these current braces work, I honestly find it hard to believe that people are even bothering with SBR-ing stuff anymore. To me, it's just not worth all of the hassle, but I understand why people do it. After all, SBRs are pretty badass. Overall, for me, NFA items are far more restrictive than pistols and that's really the deal-breaker for me.