Picked up a Colt M4 .22lr when they first came out. At first it was a little balky with some ammo, seemed to have light firing pit strikes. Even though feeding was not an issue, I tried adjusting the "adjustment screw" as outlined in the owners manual. This seemed to work, and now it is 100% reliable with most any high speed ammo I feed it, and accurate to boot. I have a Bushnell red dot sight mounted on the handle, and it makes for a great plinking gun for rapid fire drills on attacking clay pigeons and beer cans. I have heard good things about the S&W and Ruger versions as well, though I picked the Colt as it most closely imitates the form and function of a real AR M4.
Larry