Burrard discusses chamber dimensions and proof issues in his The Modern Shotgun. He observed that under the Rules of Proof there were no legal limits on chamber dimensions so the Gunmakers Assoc. drew up what became minimum dimensions. Gunmakers then typically made chambers slightly larger to accomodate faster loading. While there was some variation among makers, most increased chamber width by two thousandths and chamber length by five thousandths.

Such a long, long time to be gone, and a short time to be there.