I've only edited this like 10 times. Watch this
. I can't seem to find the exact video. The one I watched seemed to be referred by several shooters. I shouldn't try explain this in case I can't remember exactly. I would break down the gun so I had the barrel by itself. After seating a dummy round you should seat a bullet long and work your way down in length. Drop the bullet in the chamber, If the bullet is sticking where you have to pull the bullet out, then you are too long. Keep seating it deeper into the case until the bullet drops in the chamber and it should fall right back out. You shouldn't have to pull it out of the chamber. Also make sure the case length is within spec. Once you have done this several times and taken measurements you have a very good idea of where you are touching the lands. Then you back it off more for clearance.
You can't really do this with a bolt action so I use the Hornady OAL gauge then. Or I guess you could use the marker way by marking a bullet and sliding it in.
The one bullets that had a hard time chambering in the carbine was because they jammed. They didn't want to slide up and align right.
When I had a hard time with the action closing on the XDM it was because the oal length was too long.