I have not handled the .40. But I owned a 9mm version for a few years. They are a solid rifle. Works every time, pretty accurate, mellow recoil. It got me into the pistol caliber carbine thing. I sold mine and built a 9mm AR after that, then picked up a CZ scorpion and sold the AR. Not sure what the JR cost these days, but check out the Scorpion, they are a really great rig.
The Scorpions are one of the coolest looking guns that I'll never own.

I hear the guys at CloudBreak Design specialize in accessories for the Scorpion.