you are looking at this backwards Bud.......the distance doesnt matter,THE VELOCITY does.....
get a fast twist .308 and find out,or load your current rifle down till you can achieve muzzle velocity the same as your full power load would be doing at say 600 yards and have a look and see.....for sure a box of old magazines or jugs of water arent perfect...but they are consistant....always the same.
closer shots you just as well off using a cheaper option a little slower...or as you suggest a mono going like the clappers.....
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