That COAL determination is definitely wonky... both the fact that your measurement is almost certainly jammed into the lands at least a bit, and because COAL is not indicative of ogive as others have mentioned.
I came up with a method myself, which is based on
this method but doesn't involve removing the barrel. I get very repeatable measurements from it and haven't had any issues:
Prelim: Buy a tool to measure ogive if you don't have one (ensure it is steel not aluminium or anything softer). I already had one because using as a baseline for seating depth is infuriatingly all over the place.
1) Get 1 piece of brass sized.
2) Remove your bolt and insert the empty brass into your chamber, seating it with a light tap from your finger (not a tool of any kind). Lower a dowel in behind it and mark it somewhere where you can repeatedly reference to the gun (ex. back end of the receiver where you took the bolt out).
3) Insert a bullet in the brass to a length you are reasonably certain is too long (or if using a mag-fed gun, the max that will fit in the mag as you're not going beyond that even if you don't run up against the lands... I highly recommend this for mag-fed guns as the first time I did this for a gun I wasted a lot of time getting a measurement then realized completed rounds were too long for the mag so it didn't matter. But still useful to continue if you ever change bullets).
4) Gently lower the case + bullet into the chamber until it stops and apply the light tap (finger only, no tools!) as in Step 2. Gently lower your dowel on top of it. This will give you can idea how much further in to seat the bullet.
5) Seat the bullet in further according to what you think makes sense.
6) Repeat Steps 4 and 5 until your mark on the dowel is at your reference point on the gun you used in Step 2. Record this ogive measurement.
7) Seat your bullet 20 thou deeper and re-insert, it should be able to fall out freely when you overturn the gun.
8) Re-insert the bullet+case but press the case in there this time. Unless your chamber is dirty the case+bullet should still fall out when you overturn the gun, or at worst give it gentle shake.
The key to the above method is not to force the case into the chamber until you've completed up to and including Step 7. I've been wary of any methods that involve trying to close the bolt to see if the length is good, too easy to jam it in the lands.