I would try a sized, un-primed brass. Place a 85 hpbt in the case mouth. Do not crimp. Run this "dummy" cartridge into the rifle chamber slowly until bolt locks.
Withdraw slowly as not to disturb the bullet. See what you get.
2nd choice is get a 6 ARC OAL gauge.
[link]
https://www.youtube.com/watch?v=fW6H9HJzsvQ[/link]
