RB-I am no means a expert shooter just experiance out here on the prairie over the years but get 3/4 inch groups at a hundred yards fairly easy with heavier bullets.Still working on lighter ones in the 85gr range for coyotes.As for factory ammo I dont have alot experiance with it but to break in barrel, pretty expensive to shoot at $45 or more for a box.This is one round where it really pays to reload.Also helps with accuracy due to the long throats,seat the bullets out as far as magazine allows.As for meat damage,well I wouldnt go for the "Texas heart shot" if you hit solid bone in front shoulder it is devestating(throw both away)keep em in the boiler room,neck or even gutshot(not on purpose)and they go down quick.
Like I said earlier I'm no expert,but if your looking at an accumark in 257wby for deer hunting I think youll be happy.
Quick hello to Nodak 7mm,leon told me about site pretty interesting stuff.