Nah just fancy lingo , two routes to the same place.

Dellet wrote Today at 9:10am:

I have a rifle with nearly flat bottom grooves. I have never gotten a slug measurement that satisfies me. The closest is a tight wrap with a beer can and subtracting twice the metal thickness, is this the measurement I should use.
Mike

The original directions that were questioned were to “determine the cross section of the bore”, doesn’t tell you how to go about it. In theory with an even number of parallel grooves you could stick a set of calipers in the muzzle and get a direct number. Not so with an odd number.

There are a lot of ways to do it based on tools available and your sense of humor.

I generally use the twist the bullet in the caliper method, that is determining the cross section. Then size the bullet on the fat side.

Just pointing why you’ll get a bad number just stick a set of calipers on a three groove bullet and get an accurate diameter. Kind of like trying to figure out what diameter circle fits in a triangle. 

Its a very useful book filled with helpful information on trapdoor rifles. Dont dwell on its imperfections gorge on the meat.
I have had the book for 30 years and I still occasionally go back to it if I may have forgotten something like maybe the proper use of the Buffington sight with the different surplus loads or some such. 
Wanna hear a funny ?  My Dad told me when he was a young man that the state of Montana moved some problem bears out of the park and into the Bitterroot where he lived. The bears started killing cattle so F&G hired some local boys to hunt and kill the bears of which there were five.  My Dad got four of the five with the trapdoor 45-70.  He said he bought the rifle with a case of BP ammo for $5.00.  Wow ! Now the original arsenal ammo will bring that much per round.  He told me his horse got one of the bears because he was asleep in the saddle and when the horse walked up to a water troth a bear stood up that had been napping on the other side. He said his horse bucked him off and the rifle fired and when he jumped up found the bear was shot dead.  Good horse !