Yeah, barrel length really matters if you have a 9mm you're trying to keep subsonic for a can. You find out that you pretty much HAVE to use 147gr bullets. I had loaded some 125gr Sierra HPs that were subs out of 4-5.5" pistol barrels but they picked up 250FPS out of a 16" barrel.One note on pistol caliber carbines - the recoil is more than I expected. The extra barrel length allows the bullet to reach greater velocity, so, the recoil is higher. First time I fired my Ruger model 44 (a .44 magnum carbine) I was shocked by the recoil. Kicked harder than a Garand. A lightweight carbine, coupled with a powerful cartridge, added up to significantly more than I expected.
So, when I saw the rubber butt pad on your carbine, I smiled. Sure wish my Ruger had one!