noobyspud, browse around the
Spudfiles Wiki. In particular, read the
Chamber to barrel ratio page.
I don't know how old you are but this is why you've been taking math in schools for the last XX years. Math really does have useful applications and it really is a good thing to learn. I suggest you actually pay attention in math class.
Starman, actually you don't have to use the pipe radius for the CB calculation, diameters work fine. You do have to use either both radiuses or both diameters and the calculation must involve the ratio of the two volumes, as it does in a CB calculation.
Volume<sub>chamber</sub> = (CB ratio)(Volume<sub>barrel</sub>)
converting from chamber volume to chamber length for a given CB ratio and barrel diameter and length;
Length<sub>chamber</sub> = (CB ratio)(L<sub>barrel</sub>)(D<sub>barrel</sub><sup>2</sup>) / (D<sub>chamber</sub><sup>2</sup>)
Since (D<sub>barrel</sub><sup>2</sup>) / (D<sub>chamber</sub><sup>2</sup>) = (r<sub>barrel</sub><sup>2</sup>) / (r<sub>chamber</sub><sup>2</sup>) it doesn't matter if you use diameters or radiuses.