Hi.............. I'm back

So, I posted back about wanting to build a pistol for HD and car carry.  Ultimately I wanted to suppress it as well.

After going over and reading everything, it seems that I would want at least a 11.5" for 223.  

Well, my lower is marked Caliber- Multi.

So I'm about to ask another dumb question.

I was previously told that the stripped lower I purchased could be made into a pistol (That was my original question).  Now I'm asking since it's stamped multi, would there be anything on the original paperwork that specified caliber?

The more I'm reading, the more I think 300BO would be ideal.  I could keep it short (9") and still add a can.  And still have effective noise reduction compared to 223.

Am I missing anything?

Thanks and regards

If you have a stripped lower you can make it into whatever you want, you can't make it into a rifle, then go back to a pistol, if you make it a pistol, then you can go back and forth all you want, but as far as caliber, you can make it whatever suits your fancy.  If they marked the 4473 correctly, which means is should have been marked "Other"  then the world is wide open, but even if they marked it wrong, the world still belongs to you, so build what you want.

That said, always check your state and local laws to make sure those dummies didn't throw some wrench into the process.

This you should be good to go, Multi means multi, meaning multiple calibers will work with it.

Thanks.
Does it makes sense to go 300 BO?  I was thinking for short barrel suppression with decent ballistics, it's better than 223.  I really didn't want to add a new caliber because I don't really have a lot of money for the ammo.

The original option is to do 12" 223 and have less than ideal suppression and performance.  I have been reading that you really need to keep 223 at 14.5" or more ideally.