Possibly dumb question, regarding length of shell in an over / under. Ruger Red Label, marked for 3". Winchester doesn't make a 3" target load. Does it really matter in an over / under if you use 2-3/4"?
No, they don't make any 3" target loads because of the expense and the recoil. You'd get beat up pretty badly after shooting about 100 rounds of skeet using them. You can use 2-3/4 in 3" guns but cannot use 3" in 2-3/4 " guns.
That ruger o/u is a great shotgun, the only time you might want to shoot 3" shells is if you are hunting ducks or geese or turkey. 2-3/4 " shells are great for all your target practice and are easier on your shoulder and easier on you pocket book.