If you are shooting a factory gun that's good shooting. If you are shooting custom gun and working the load you may only want to neck size and play with load. That's still a good group at 400 yards. Your shooting .88 at 300yards and if that's factory that's good. Custom triggers with around 1lb pull will tighten your shoot group if using bipod or sled on front and you need a sand bag on rear of your weapon.. Sounds as you are already a good shooter. What weapon and load are you using? Keep putting them down range at 400 with good optics and a clean gun and you will get tighter groups.
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