A) When switching barrels you should have the headspace checked by a gunsmith and it could require machine work.
B) If you just out a different factory barrel on, it could be just as bad.
C) If the barrel looks smooth when you look down the bore, there’s a really good chance that your accuracy issues lie in the chamber job, if it’s the barrel at all. Often times, having a barrel “set back” by at least the length of the neck(more is better if there is room) will turn a barrel with a bad factory chamber into a better shooter than 9/10 factory barrels.
D) I’d rather have a .260 than a 7-08 unless I planned on shooting big animals at close range. Yes, a 7-08 will shoot things at long range very well. It’s just that you’re using the exact same case to drive the bullet, and a 6.5mm bullet beats a 7mm bullet if they’re the same weight. A 168gr 7mm has a slight edge on a 156gr 6.5mm in terms of taking game. You can go up to 195gr with a 7mm if you have enough twist. If you’re going to shoot heavy bullets the advantage is there. Will a 7-08 kill lots of animals very well with a 140gr bullet? Of course! But a .260 with a 140gr bullet is even better.(Higher BC for the wind, and higher SD)