the length posted on a box of shells is the fired length of the shell so it clears the breach on extraction.
per a post on www.shotgunworld.com "The linear dimension used to identify a particular shot shell refers to the length of its fired case and not that of the loaded round. Actual length will vary slightly among various brands of shells, but those designated as 2 3/4 inches usually measure around 2 5/16 inches before firing and 2 5/8 to 2 11/16 inches after firing.
Three-inch shells usually measure from 2 1/2 to 2 5/8 inches before firing and lengthen to just under three inches when fired. " [/b]
I have a few different boxes from Winchester , 12Ga low brass upland 2 3/4 in 4 shot ... and Winchester 12Ga high brass duck and pheasant in 6 shot both measure 2 3/8 from top of shell to bottom of brass. and my shotgun will shoot 2 3/4 or 3" shells stamped on the barrel. i have not shot in 18 years . i have only had a feed issue with a few factory shells. they had a bad fold and had a pucker pinch on the top outside lip, that would get snagged up on loading. a quick shave of the plastic with a pocket knife and the shell loaded good after it's poor crimp pucker was knocked off. that fat lip would stop the shell at the mouth of the chamber.
i did have a buddy that his new gun did not like high brass. shells would get caught up some times 9/10's of the way into the chamber. my gun did not care it would shoot both high or low and any brand shell all day long. and another that his gun did not like extracting "don't recall the brand of shells" low brass 3 in/20 would pull out 1/2 way and stay in the chamber / block the new shell .. he just had to buy brand X .. and he had no issues .. finicky extractor lol .. i have seen guns not like a brand of ammo, rounds were too fat and the tolerance on the inside of the chamber were on the "tight side" so any round on the high end of the spec. would be a snug fit or would not load at all. and i am talking about factory ammo not reloaded stretched out shells.
you stated "wont feed thru my 870" are the rounds getting stuck as soon as the action comes up to close the chamber? like the top of the shell is too fat or like the shells i had to shave a bit of plastic off because the top had a pinch bump that would catch and keep the round from entering the chamber?
then you say you measured these are just over 2 1/2 inch long. you sure your not trying to put a 3" shell in to a 2 3/4 in gun ?? that shell @ 2 /12 in length may be a 3 inch shell in the wrongly marked box / wrongly marked / used brass bottoms?? your shotgun should be stamped 2 2/4 or 3" or even state will accept both 2/3/4 or 3" shells if it does not state both or only states 2 3/4 i would not shoot a 2 1/2 inch AKA 3" fired length shells in it.
firing the wrong size shell could be catastrophic. as stated by SemperFiGuy Might be useful to contact S&B Customer Service and tell them what you have.
If the shells are marked wrongly, they could also be loaded wrongly. <-- my thoughts exactly
