those cases should be of identical sizing (the cartridge case, not the cartridge overall length, or C.O.A.L.) measure the length of each round with a micrometer and report back (although they may be the same length, the 165 gr bullet is longer and may either be seated out farther or the ogive of the bullet is farther forward, hanging up on the rifling.) That said, just your comment re: the neck-i'd return the ammo and switch brands. I do favor the 165s in .30 as they have better BC/SDs. Good luck! The only H&R I ever had was a single-shot. Is yours an old, sporterized mauser made for one particular load?? measure the neck diameters, too and compare. Your barrel/chamber may be picky with respect to dimensions. A chamber cast (very easy) will also tell you a lot about the gun. Is the one load crimped into a bullet cannelure and the other not (causing it to bulge or flare at the neck somewhat?) typ. Rem ammo is cheap, but made to spec, but anything's possible (in mass production-just look at car recalls)! was the gun re-barreled?