No kidding. Some things just make you go 'duh'.

While 18.7 may be a tad tight for the chamber, SAAMI drawings show shell diameter as .744 with a tolerance of -.009.

The chamber is likely marked to the 'most significant' tenth of a millimeter.

A shell at SAAMI minimum should chamber.



"The price of good shotgunnery is constant practice" - Fred Kimble