If you use an unmodified bolt, you need to use shorter 9mm hammer to clear the bottom of the bolt. Ramped bolt allows the use of regular hammer so if you want to use the same lower for .223, you can simply swap out the upper and the 9mm buffer.
9mm buffer is heavier and required for more reliable function of your 9mm carbine ($15 at PSA) -
http://palmettostatearmory.com/9mm-steel-body-buffer-assy.html
I have 17" Just Right carbine, built two 16" PSA 9mm carbines from MOE kits and recently ordered 16" PSA 9mm upper with keymod handguard. All current PSA 9mm uppers and complete carbines use the ramped "hybrid" bolt which is compatible with AR magwell block or glock based lowers.
The differences between JR and AR/PSA carbines are:
- JR carbine uses 1:16 twist rate barrels while PSA and most AR carbines use 1:10
- JR carbine uses modular magwell and can be converted to 9mm/40S&W/45ACP using caliber conversion kits
- AR/PSA carbines use more typical mag catch on the right side of the receiver but JR carbines use mag catch on the left side of the receiver
- JR carbine can use Glock, M&P and 1911 mags with magwell change while AR/PSA carbines use Colt or Glock mags depending on the receiver used
- PSA uppers do not use brass deflector and eject spent brass without issues for me (I use Caldwell/UTG brass catchers and they work well with JR and PSA uppers).
I am currently range testing how much 1:16 vs 1:10 barrel twist rate has on accuracy at 50/100 yards and if rain stops enough for a range trip, I could have a report by tonight -
http://www.thehighroad.org/showthread.php?p=10228100#post10228100
PSA has complete 16" 9mm uppers for $399 with front sight and $429 with keymod handguard -
http://palmettostatearmory.com/ks-47-ar-47-ar-9/ar-9.html
You can then either order the PSA Glock lower or use AR lower with magwell block to use with Colt style stick mags (along with heavier 9mm buffer). If you are going to order Magpul flip up sights, I would order the 16" upper with keymod handguard for freefloated barrel.
In the carbine load thread, I have found you need to maintain supersonic speed to target or your bullet may experience transonic effect which increases group size. I am currently testing lighter 100/115 gr bullets to keep them at supersonic speeds to 50/100 yards to avoid the transonic effect. So far, both 1:16 and 1:10 twist rates have stabilized the RMR thick plated bullets well and getting round holes on target around 1350 - 1450 fps (will be testing 1500+ fps today).
If you are shooting to 25 yards, light to medium pistol loads should be fine. But if you are shooting at longer 50/100 yard distances, expect more bullet drop and vertical stringing with slower target loads and may want to consider shooting faster lighter bullet loads.