[Unclenick]

MG,

The math of tolerances match with their names. If it is ±0.1 grains, it means whatever weight the scale says you have, add 0.1 grains (the plus) to it and the result will be the upper limit of possible measurement error, and then take the weight the scale says you have and subtract 0.1 grains (the minus) and that result will be the lower limit of possible measurement error.

When you look at SAAMI drawings, you will find the linear tolerances are unilateral, meaning they only go in one direction. For chambers, linear dimensions have only a plus tolerance, meaning the value given is a critical minimum and nothing in the chamber can be any smaller than that, while for cartridges linear dimensions have only a minus tolerance, meaning the value given is a critical maximum above which you cannot make them any larger. This system assures cartridges can chamber.