if you be shooting 100 yard targets, and nothing else, a .223 remington will be fine. If you ever want to hunt with your rifle, and I'm assuming you will or you would be elsewhere asking this question, get somthing with a little more poop. As stated above, a .243 or 7mm-08 will be great hunting guns but with a little more recoil, which is virtually not a consideration with the .223. The AR-15 fires a .223, in stock form, but Armalite will build these things all the way up to .300 WSM (!) But my guess the rifle you fired was a .223.