Gunsmithing Understanding Body Taper - Help

[surgeon260]

I need some help understanding how to determine body taper from a reamer print. Consider the following fictional dimensions from a fictional reamer print:

-A (Length from bolt face to body/shoulder junction): 2.000"
-B (Chamber diameter at bolt face): 0.500"
-C (Chamber diameter at body/shoulder junction): 0.490"

Given these dimensions, my intuition is to calculate the body taper as (B-C) / A = 0.005", or 5 thou of taper per inch.

Here is where I get confused. There is a table in the bottom right of the reamer print that summarizes taper in terms of "TPI in 20 inches." Let's say in our fictional reamer print the table says:

Body TPI = 0.050"

I don't understand this number. If I multiply my calculation of body taper per inch using the actual dimensions from the chamber by 20, I do not get 0.050. Rather, I would get 0.100. The Body TPI from the table seems to disagree with my calculated Body TPI by a factor of two.

What am I missing? Where is this factor of two coming from?

 

[Rocketvapor]

Is the taper in the print using the reamer centerline as the data?
Half taper on one side, half on the other?

 

[surgeon260]

Another way of describing this observation is that the Body TPI (taper per inch) value in the case taper summary table is always 10x greater than the Body TPI (taper per inch) calculated using the (B-C)/A method described above. I would expect the difference to be 20x since it is labelled "TPI in 20 inches." This observation has been true for every reamer print I have seen.

Clearly there is some sort of convention here that I don't understand.