Cheapskate measuring absolute max OAL for a specific bullet...
I hope I can explain clearly enough. NOTE: As our buddies said above, all different bullets have different nose and ogive shape, one-size-OAL-fits-all doesn't work. You gotta remeasure when you switch to a different bullet.
Using two dowels and a razor blade.
Make sure chamber is empty. Close the bolt. Run a dowel down from muzzle until it stops against the bolt face. Mark the muzzle on the dowel with the razor.
Now remove the bolt, drop your bullet into the chamber. With a different dowel inserted into the chamber, gently hold the bullet against the rifling lands. Now put the first dowel down into the bore until it stops at the tip of the bullet, and mark the muzzle on the dowel again.
The difference between the two marks is your absolute maximum OAL for that particular bullet.
You can use a cleaning rod or other metal rod and a marker, if you can mark to the nearest 0.001". I can't do that with my old eyes, but perhaps you are accurate down to the 1/1000 inch.
The tools made by Hornady and others do the same thing, this dowel method is just for us frugal cheap-o cheapie cheapskates who want to do the same thing at no cost.
Plenty of good folks will tell you to stop being cheap and just buy the tools. Just smile a big grin and nod your head in agreement. The tools work great. So does the wooden dowel. The gun doesn't know the difference, it will give you the same measurement either method.