http://www.shooterscalculator.com/recoil-calculator.php

270
150 grain bullet at about 2800 fps with about a 55 grain powder charge

12 ga


Therefore (rounding off to the nearest 1/8oz -- rounding up when it is not very close to lower number):

~ 6 pellets OO = 330-grains = 3/4oz. load
~ 9 pellets OO = 495-grains = 1-1/8oz. load
~12 pellets OO = 660-grains = 1-1/2oz. load
~15 pellets OO = 825-grains = 1-7/8oz. load
~18 pellets OO = 990-grains = 2-1/4oz. load

~ 6 pellets OOO = 420-grains = 1oz. load
~ 8 pellets OOO = 560-grains = 1-1/4oz. load
~10 pellets OOO = 700-grains = 1-5/8oz. load
~12 pellets OOO = 840-grains = 2oz. load
~14 pellets OOO = 980-grains = 2-1/4oz. load

powder charges generally run close to 30 grains
velocity can vary but 1200 fps 1400 fps is common use the calculator, a 270 will generally have less than 1/2 the recoil of a 12 ga buckshot load