I'm about to go down the 9mm rabbit hole, I'm not doing another SBR due to the new regulations so SBR of a dedicated lower is out.  I'm building a a small compact light 9mm and I'm trying to decide if the best route would be to dedicated a current SBR lower to a mag block, or  get a pistol mag dedicated lower.  Basically is a SB or shockwave going to be ok or do I need a stock?  I have a shockwave on a 300BO right now and its ok for low volume shooting, but the 9mm will probably get shot a lot so is a brace going to be enough?

I can shoulder my 9mm PSA upper fine with just a longer KAK buffer tube (designed for the SiG brace) and no shock wave orSiG brace on a PSA dedicated Glock lower... I do have a Macon Armory mag block for use on my registered SBR for USPSA competition... USPSA does not allow pistols to be used in the PCC division... a LAW folding stock adapter with a pistol length buffer tube is about 3/8" shorter than the fixed length KAK tube... the PSA 9mm lower comes with a 5.4oz buffer.. I shoot 146 power factor reloads, and all day with just the tube and possible a rubber end cap might not be that much fun...not bad for a couple of 30 round stages... the blow back 9's have more felt recoil to me than the 5.56 does

I have a couple SBR's but run my 7" 9mm as a pistol with the shockwave brace.  The brace doesn't bother me to use at all and it's probably one of my funnest guns to shoot.  Plus as a pistol I can through it under the backseat of the truck and take it with me when I travel if I want.

I will probably SBR it down the road, when I switch from a mag block to a dedicated 9mm lower though.