My 338 win mag Bar kicks less than my .243 Win, not sure how much the auto vs. muzzle break contributes tho.
I've done a lot of research into mercury brake and have heard nothing positive about them, except from the vendors.
My brother, nephews and I subjectively report that my Big Bertha SA 50 BMG kicks the least of all my guns, followed by the 338 BAR. My nieces who weigh less than 115lbs say the 50 BMG kicks harder than the BAR - but they enjoy shooting both guns. Big Bertha does push you back.
I wear muffs and ear plugs at the range, electronic muffs if I have to hunt with a brake.
I'm having Kirby put Holland QD's on all my RUMS - his customers have had good luck with them and I've read good reports on them.
At the range I also use a strap on (shoulder recoil pad). Midway had them on sell for $5 each last week, so I bought 3 extra.