I have a spare .223 upper receiver laying around. Is there any problem with using it as the basis for a 9mm upper half ? As a right hander, will I get gas or brass in the face without the 9mm deflector installed ?

I plan on installing a 10.5" 9mm barrel (RRA ?), a ramped 9mm bolt assembly (CMMG ?) and a top-loading mag block (Hahn ?). When installing the barrel, I presume simply applying proper torque is sufficient to keep the barrel nut in place ? My SBR'd lower already has a chopped M16 hammer and a 9mm buffer installed. My hammer spring is currently modified per the 15 minute trigger job (i.e. one leg removed); should I go back to an unmodified hammer spring to apply the proper resistance to the bolt carrier ? Lastly, are there any other special considerations to ensure reliable functioning, or does a 9mm build pretty much go together like a .223 build ?

There is absolutely no reason why you cant use a .223 upper for a 9mm upper build. The only dofference is the 9mm ususlly has the deflector and shorter door which makes no difference at all. I run one and it is flawless. The ejected rounds still fly straight out to the right and I get no gas to the face either. I did plug the front gas tube hole so it doesn't spew gas and [powder residue forward under my float tube which is a pain in the a__ to clean out. Oh their ususlly cheaper compaired to the same thing for the 9mm because your not paying for that stupid deflector and shorter door.

You can probably use the QD deflector from CMMG.
Dave N

Thanks. How did you plug the gas tube hole at the front of the upper receiver ?

if i remember correctly, someone makes a plug for the gas tube hole.

You remember right, and for the life of me I can't remember who it was. I seen it years ago when I was going to plug my 22lr A2 upper but never ordered one.

WildMan! That was post 223! you used that one up and now it's gone forever

I didn't bother to block the gas tube hole on the .223 upper I used.  No suppressor (can't have them where I live) and a 16" barrel (no SBRs either) so very little gas blowback.  I do have the little 3/4 door and the plastic gas deflector, for the "9 mm look".

I've been very satisified with my 9 mm AR set up.  Best of luck with your project.

Not legal advice, MHO, YMMV, etc.

Hey guys...Plug it yourself......I built a jig to hold my upper in the upright position. On the inside of the upper there is a recess that is drilled about a 1/8" deep. It's a 5/16" dia. recess. Drill straight thru, and from the outside above the barrel threads tap it for 3/8-24 and install a set screw. I used a 12" long drill bit. I installed a 3/8-24 x 1/4" set screw and installed it witlh red lock tight. It's super easy and takes 5 minutes to do. Of course you can never go back to a 5.56 upper again without either drilling or heating the upper up to remove the screw.

I did this to give the look of a standard 5.56 upper. The Fwd Assist is just along for the ride. I added a 9mm deflector and port door to complete it.

Use whatever upper you have.. all you need it to do is hold the barrel and provide runway for the bolt... everything else is decoration.

I currently use the old A1 I had with 9mm barrel using std front sight base,, had a fella give me a gas tube that was burned out, cut it short so that all it does is pin into the sight base, and fit into the upper,, but not inside the operating area.

I have a 9mm smooth sided stripped upper for sale (Equip Exchange) if you're interested.