Hello,
You have not forgotten anything from your sixty years of shooting, just like me and I congratulate you for it.......
All the French weapons and mainly the old military weapons after the Revolution of 1789 are in millimeter and it is an exact fact ......
We also have our French "grain" that, by my job, I used as well as the "inches" (27,07mm instead of 25,4mm now) which are French measures now little used except in certain trades of which mine: I was a watchmaker-jeweler, thus....
....
The grain is theoretically an old unit of mass representing a seventy-second of "Gros", worth about 53.114 milligrams in France (division of the Paris pound). It is approximately the mass of a good grain of barley or wheat. For pharmacists, the scruple is worth 24 grains.
In jewelry and watchmaking, the grain is still used in its decimal definition worth 49 milligrams (about 1/576 of an ounce of avoirdupois or a little less than a quarter of a carat), to express the weight of batches of pearls or oriental diamonds.
This aside, I used the expression "American grain" because we are on an American forum, but I could be wrong, and on an English forum I would have said "English grain"...
To come back to weapons, it is true that the military culture imposes/ did impose a particular language, for example a .50 caliber in the army becomes a 12.7mm caliber...
For the rest of the population and the civilians that we are, a .50 caliber is a .50 caliber, a .22 caliber is a .22 caliber, a .45-70 is a .45-70, a .45-70-500 is a .45-70-500, etc..., etc..... Never millimeters in calibers: always fractions of an inch and grains.
Never a question of powder weight: always grains. Imagine, just for fun, a 45mm caliber loaded with 70 grams of black powder or a 45mm caliber.
The military used to say, a long time ago now: caliber 11.43mm for caliber .45, but that was a long time ago and only for military weapons...
In short, apart from these few particular cases, the good people always give, as far as weapons and ammunition are concerned, the caliber in fractions of an inch and the weights in grains, which is logical since calibers and weights are given in fractions of an inch and weights in grains since the beginning of their existence.................. except for a few rare guys diehards in the process of disappearing for whom it is always necessary to make the conversions in metric so that they understand, but there, it is another history.......
PS: you will certainly find a lot of mistakes that need to be corrected and I apologize, this explanation in a language that is not really my native language has been for me quite long and complicated, in general I do shorter, with as many mistakes, but much shorter ....