>What kind of ammo is that, now?
Yes, you nailed it. A .220 swift and very few others (6mm Rem, .17 Rem Fireball, .243 Rem, .243 WSM, .22-250, .223 WSSM, and a lot of wildcats. Like Ackley's wildcat Eargesplitten Loudenboomer... 4600 fps...http://en.wikipedia.org/wiki/.22_Eargesplitten_Loudenboomer
Most of those factory rounds are not common because people get tired of shooting out a barrel in 2000 rounds... That is about all of them... Not a .223... at least not unless handload it with a 36 gr bullet a powder slow enough that you have to put it in with a drop tube...
My rule of thumb is that everything about guns in movies is either wrong, impossible or ridiculously dangerous.