Ke32,
Welcome to the forum.
The 2.860" COL is merely the SAAMI maximum that you cannot exceed without risking the cartridge might not fit into some rifle's magazine.
The SAAMI drawing shows a range of 2.560" to 2.860" for normal bullet shapes. So the SAAMI standard maximum may not be the correct COL is correct for your bullet. Berger should list a COL for the individual bullet that is different from their other bullets. If they don't (I don't have their data) then call them to ask for one.
Below is an exaggerated illustration showing two different bullet profiles seated to the same distance off the lands of the rifling (the jamming clearance, so to speak). But because of the profile differences, the COL's have to be very different to have the same throat clearance. This is the issue you are running into.
To get a ballpark figure, pick up a 3/16" brass rod at Lowe's or if your cleaning rod is a Dewey with a male thread you can use it. Remove the bolt from your rifle and stick one of your long cartridges into the chamber and press the bullet up against the land by pressing with your finger on the head of the cartridge. Hold it there while you run the rod in from the muzzle until it contacts the bullet tip. Use a pencil to mark the rod where it is flush with the muzzle.
Push the cartridge out with the rod. Put the bolt back in and close it. Run the rod in until it stops on the face of the bolt. Mark it with a pencil where it is flush with the muzzle.
Remove the rod and measure the distance between the two marks. This is the COL at which your bullet jams the lands. Since this won't be a super precise measurement and since bullet lengths vary some, I would subtract 0.030" from the result and call that your new starting COL and work a load up with that if Berger doesn't tell you anything different.
In making this measurement, the more flat and square the end of the rod is, the more accurate it will be. Just be sure the ejector on the bolt doesn't fool you about where the bolt face is located. If you want to buy a tool and get more precise measurements, that's fine, but this should get you in the ballpark.
