can i use a 5.56 lower to build a 9mm ar on?

what if i sbr the lower?

Yes you can. No problem, I have built a couple. One of my most favorite rifles is a 9mm upper on a Cav Arms 5.56 lower.

SBR? Yes you can!  Fact is, you can SBR a lower and them switch its caliber. Fact is many do this for just that reason. They can do one stamp and then swap out uppers to make their SBR the caliber they want. Its the Lower that gets the stamp.  Its a good way to do it.

MORE TO THAT STORY!!!!!

Lower MUST be marked "multi" or labeled for every caliber you intend to put on that lower.  Otherwise, and you are going to prison!

P

Cite your source for the part in bold.  I call BS.

Well, then the feds need to build a few more prisons as a ton of us do this everyday. See you all there and we can start up our own gang!

Edit to add: DON'T look at my Avatar Mr ATF.

Seems this question comes up a lot. Got this from  "The High Road" forum:  

"TexasRifleman
April 3, 2011, 03:04 PM
Whatever lower you decide just be sure to use one marked "MULTI" for caliber. On your Form one it is going to ask what caliber. If you register your SBR on a 5.56 marked lower, well.......what if you want to put a 9mm upper on it? Using a "MULTI" lower for an SBR just makes good sense in my opinion.

ATF is no longer accepting Form 1's marked "Multi" or "Various" etc for SBRs. They require a specific caliber be on the form. They seem to still be accepting Form 1's with more than one caliber written in however. Same with length. It used to be common to put "less than 16 inches" on the form but those are now being rejected.

That said, Tech Branch has issued written opinions that say it doesn't matter what is on the Form 1 caliber or length wise. Basically once you get the tax stamp back you have an SBR and the tax stamp allows you to possess the firearm in ANY SBR configuration, regardless of what's on the Form 1.

The issue seems to come in if you decide to sell the rifle as a complete kit you will need it to be in the same condition as the Form 1 says, length and caliber. That is an ATF agents verbal opinion to a dealer I know, I have never seen that one in writing.

There is no law against having an SBR in a configuration that does not agree 100% with the Form 1 as far as length and caliber go.

The same is true of Title 1 firearms. If I own a 24 inch rifle and I cut the barrel down to 20 inches, that does not change the Title 1 status of the firearm and I don't have to notify anyone.

No difference here. A 5.56/11" SBR can be turned into a 9mm/7" SBR without any paperwork. It's still an SBR.

Tech Branch has also said that the caliber markings on the lower do not have to match the caliber of the installed upper. The Form 1 just wants to know what is engraved on the lower for identification purposes. Again for a Title 1 firearm this would be the same thing. Reaming out a bolt action rifles chamber to accept another caliber does not require the markings to be changed. Putting a 20 inch 6.8 upper on a Title 1 lower marked 5.56 is perfectly legal."

And, there is a recent ruling from BATF that deals with converting a pistol to a rifle and then back to a pistol. BATF says "no problem".  While it does not directly address this issue it does make reference to a matter close to it, that is converting back and forth between "rifle" and "pistol". I am not an attorney (nor did I play one on TV) but since much of this started with the Thompson Contender there is a precedent here. After all, what caliber is the Thompson? Oh, whatever you want it to be. Seems they are not concerned with caliber, rather they consider "configuration" when it was built or registered.

In the end I wonder if you can find  a clear cut definitive answer to this in writing.  BATF doesn't address it on their website and I find it hard to locate anything in writing directly from the BATF. I see lots of people telling various stories about how this worked out for them. Seems you ask different the same question to different people at BATF you get different responses. Here is what one poster said"

"So, I called the NFA branch in W. VA this morning and pitched the question to them. She told me that (on a Form 1) Question 4c must have one caliber, 4e must have one bbl. length, and 4f must have one OAL. Additional calibers, bbl lengths and OALs can be entered into 4h. Additional Description.

She answered my question very quickly as though she had been asked it frequently. Also, she was very polite and friendly.

It would not surprise me if some folks (or a lotta folks) have done it differently and had it approved. I'm going to follow the instructions from the NFA branch, and I thought I'd share what they said. "

Endless....ask five people, get five different answers.