I believe that only penetration THROUGH the brain is of any value with larger critters, and it's my experience that for "normal" springers a .177 cal pellet will trump a .22 cal pellet when it comes to penetration.
IMHO, one of the problems with the term "50 yards" is that it's very difficult to shoot a pellet accurately enough under actual field conditions to make the required surgical hit.
I don't know how tough a Nutrea is compared to raccoon or wood chuck but I've dispatched several of each with my .177 R9 and the relatively deep penetrating hard lead CPL(compared to other mid weight pellets).
Back of head shot at close range
Just behind base of ear hit (see parted fur) at about 30 yards.
Bottom line, all my bigger stuff has been taken at 30 yards of less using head shots.