I would go with the 55.5 grain load and if I wasn't happy with the grouping, ay with seating depth. I would shoot 3 shot groups at each seating depth, maybe 4 if you want to get carried awaywith plenty of time between each group. 18 is not a large extreme spread to me, perfectly adequate for shooting very far.
It's a hunting rifle and you only want to use it out to 300 meters right?
So how tight do you need the groups to be?
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