Help understanding SAMMI drawing

[Papa Charlie]

I was looking at a SAAMI drawing for the .308 Winchester and was trying to understand how to determine Freebore. I copied the drawing below and I assume you subtract the dimension in the cartridge drawing (2.015") from the chamber drawing of (2.3025"). I drew lines to the dimensions I am referencing. Am I reading this correctly? Your help would be greatly appreciated.
Thank you.

 

[SG4247]

I believe it is 0.090" .. derived from 2.1388" - 2.0488".

Very hard to see on my screen. Looks like the lands start at 2.1388" and taper down at 1 degree-45 min to the diameter of .308"/.300" at 2.3025".

Edit: FYI..the case drawing does not define free bore, only the chamber drawing.

 

[243winxb]

Free bore .090"

Throat is 2.3025 - 2.1388 = .1637" Throat, Also referred to as Leade or Ball Seat and is associated with Free Bore.

Go to the SAAMI glossary for definitions.

 

[Papa Charlie]

Reading SAAMI:

"FREE BORE
A portion of the chamber, usually cylindrical, forward of the casemouth of a diameter larger than the projectile in which rifling is not present. See Bullet Jump. See Throat."

So wouldn't that be from the 2.0488" dimension to the 2.3025" dimension as the diameter is 0.310" between these two dimensions? The rifling looks to start at the 2.3025" dimension line.

 

[SG4247]

Nope, rifling begins at 2.1388". There exists a plain cylinder rearward of that (free bore), for the .308" dia. bullet to occupy. No lands there.

Imagine you are the bullet facing a ramp (land), that ramp begins at 2.1388" @ .310" diameter and that ramp goes up at 1 deg 45 min angle decreasing the land bore diameter to .300" at 2.3025".

Looking from the side of the chamber, think of it as a ramp with legs of .1637" long x .004" high (half of diameter .308" - .300"). This ramp is otherwise know as "throat". The ramp (throat) represents where the land is cut at a very shallow angle, to allow the bullet to be engraved into the rifling with "relative" ease.

 

[mbappe]

So the lands would not narrow down to .308" until 2.1388" + .1637" x (.002 / .010) = 2.17154"?

What about the grooves? Do they begin their diameter descent at 2.1388" also? So we have lands and grooves appearing even before the bullet is being squeezed? Or do the grooves only begin to appear once the lands have reached their final diameter of .308"?