Re: 115 Sierra MatchKing in a 270 win
Are these cases fireformed or brand new? If shot from another rifle other than the one you are trying to chamber it in, try pulling one of the bullets and re-size the case. Before you load a bullet into the case, see if it will chamber.
If it chamber without a bullet seated, try seating one. Sounds like a case/neck issue not bullet seating issue.
I have had similar issues. Before I put any bullets into the seater, I chamber to see if the cases will even fit.