My department has four of them, and they are truly scarey. I fired at 1/4" angleiron, and it looked loke someone had cut holes in the metal with a torch. I don't know about the terminal effects, but it is DEFINITELY armor-piercing. The bullet is supposed to tumble 180 degrees in the target and provide a non-survivable wound from any torso hit. At least that's what we've been told. It's also supposed to have very little penetration through wallboard, but I haven't shot that yet. It is LE and military only.
There is no difference in barrel length from the subgun and the pistol. the subgun has a suppressor that can be attached to the barrel in place of the flash suppressor. It is about 6" long by about 1.25" diameter. there is an AP round, a tracer round, and a subsonic round. There is NO felt recoil when firing, and a fifty round magazine can be emptied without any appreciable change in point of aim. we have aimpoints on ours.