If you feel like it is ammo related, it might be helpful to run a few rounds over the chronograph and pick the stoutest one for your testing so you know what you have beforehand.

I have always found S&W autos not only function better but also shoot more accurately when they are "slammed around" by the ammunition they are using. HS6 always produced the best accuracy results in S&Ws for me, and it produces velocity! I'd be looking for ammunition that actually produced original 9mm Luger ballistics, which was a 124 at 1145 FPS, if memory serves.

When I noticed that my HS6 handloads consistently produced best accuracy in several different S&W 9mms (5-6 of them) I wondered if this was a peculiarity of the cartridge or the guns. I expanded the testing out to include SIGs, and did not have similar results. My SIGs preferred Win 231 in handloads. I guess that little tidbit is not relevant here, but I always thought it interesting.