I've had everything from 1-4's to 8-25's on SPR's/MK12's. If most of your shooting is from the bench, get higher mag. If most of your shooting is dynamic, then get a 1-4,6,8,or 10. I currently run a 3.5-15 on my SAM-R, an ACOG on a LWRC, and a 3.2-17 on a MK12.