I just bought a new 550 American for my wife . I loaded some Norma brass with 125g Nosler Partitions at around 2550 fps. at the range I could not close the bolt ??? I had measured the chamber at 3.147. I used the speer data for modern 6.5x55 and seated the bullets at their stated 3.000. What I found later by loading some "dummy" rounds at various COAL's The longer the Coal, the better they would chamber. I also polished the "Claw" extractor. as a round tries to slide up into the bolt face, it would bind up on the extractor, by seating the bullets to a coal of 3.120, the problem was solved. Look at your fired brass, if the cartridge head has a cut where the extractor dug into it, then you are in the same boat.
One would think that this problem would be obvious when the rifle was test fired???
I used a 3/8 ceramic rod to polish the razor edge off the extractor, it no longer cut's into the cartridge head.
Good Luck!