...I just shot the P90 5.7mm this afternoon. I expected it to be a major disappointment. What an absolutely fantabulous piece of work! I fired a lot of stuff in the army, while doing ranges; this takes the cake. No recoil, accuracy on par with the M4 carbine, excellent operation. The 5.7mm round is super flat in trajectory, high penetration, with yawing effects after entry. Better than the 5.56mm, IMO. For you Stargate fans, there's a civilian one out -- the PS90.
The handgun made for the 5.7mm is just as good -- more accurate than anything I've shot, with better range. Little recoil (but a hell of a muzzle flash!) 20 in the mag. It was so good, I went out afterward and bought one.