I concur with the use of the 223 Rem since he already has proven he can shoot it well. I also suggest the use of the 62 gr Winchester PP as it is a proven hunting bullet. This mulie was taken down with one shot using it out of a 20" barreled .223/5.56 AR with a 9" twist. It was loaded to 2900 fps over AA2460 powder.
Attachment 294041