56" to 70" is pretty much the same as far as I'm concerned - one must work up a load and stick with it. It takes more powder to shoot further is all. I have a 66" in the 14bore and it will shoots identically, accuracy wise from 100 to 200yards with from 140gr. to 200gr. 2f GOEX - that is from 1,500fps to 1,700fps.
I prefer square grooves.