I need some help selecting the front post and rear sight aperture dimensions for NRA - DCM High Power service rifle competition. It's for a Bushmaster DCM-XR rifle I'm purchasing for my son under their junior shooter promotion. He is a beginner, and is progressing well. Bushmaster offers a choice of: 036", .040" , .046" , or .052" diameter aperture for rear sight. The Competition Front Sight, ground on three sides, is available in two widths .052", or .062". What factor's influence the choices and what is recommended for shooting out to 600 yds?