Originally Posted by RT2506
I have no idea why your groups are different but my buddy found out the same thing. This was when Remington first came out with the 260 in a 700. He got the SS with a 24" barrel. You could not hardly find any ammo much less cases at that time. He did find ONE box of ammo and was using these cases to reload and work up loads. There was not any load data then either. He was using H4350 and Sierra 120 and 140 gr bullets. He had a new bag of Rem Nickle 243 cases so he decided to neck up a few and try them. His groups went down to bug holes and he still has never went back to 260 cases.
Are you using the same brand of brass? Brass being equal, about the only difference I can think of is you will thin the neck somewhat by necking up the .243 which could cause a different POI. You might also shorten the brass length a little as well? As far as accuracy goes, it just might make a difference in what your particular chamber likes?..........Rich