Black Powder Calculations

So I read on the Internet forums and while I was at the range today helping a guy sight in and shooting the bull I remembered this formula.
To understand powder and ammo you have to read a lot of old inventions. Pick yourself some old reloading manuals say at least 50 yrs old. These will help you understand ballistics and reloading better then all the new gizmo's offered today.
BPB

You can use a formula to calculate the maximum amount of powder your barrel can burn.
It is 11.5 ( grains) per square inch of barrel bore.
To run the calculation, take the bore diameter, say .50, for a 50 cal.rifle, then divide that number by 2 to get the radius.

.50 divided by 2= .25Square the radius,

.25 x .25 = .0625Now multiply that times Pi, or 3.1416,0625 x 3.1416 = .19635.

Multiply this times 11.5 grains per cubic inch:.19635 x 11.5 = 2.258

If your barrel is 36 inches long, multiple the length of the barrel - 36inches -

times the number above. 36 x 2.258 = 81.3 grains approx. ( 81.288).