The very sharp step to the chamber was of course tried very early in the breech loading game & even quicker abandoned as totally undesirable. The cerro-safe method of course is the most revealing, but the 6" scale with a little practise is extremely useful & can be dropped (or clipped) into a shirt pocket & carried where-ever desired (mine is a constant companion). Chamber dimensions will not vary near the extent of bore dias, but ±.002/003" over the last 1¼ century or so is not uncommon. Some older chamber drawings called for about a 5° angle off the C/L for the forcing cone angle. On the approx .070" drop from chamber to nominal bore this would give a cone length of about .400". A gage +.003" from the juncture dia will stop .600" short, one -.003" will enter the cone .017". Of course on a longer cone (less angle) it will enter further, but becomes less critical. If one simply doesn't believe Burrard, Thomas or Bell on importance (or lack thereof) of exact chamber length, simply make your gage to -.003" from nominal dimension & if it enters to about the same length as your fired cases, "Live Happily Everafter", shotshells in chambers aren't precision fits.
