Jim Keenan and
Jim Watson posted very good replies as always -
but - they forgot one.
Leastwise in my mind anyways...
Because some of us OLD folks were publik skooled in the South and never took a shine to Metric Measurements
Seriously- I was raised into a work, and Metric is what is used for quite a bit of it.
Imagine me, as a little Southern Brat and hearing: 9mm, 45ACP, 20 gauge, 2.8mm ball burr, #80 drill bit, B&S 24 ga wire gauge, .22 rim-fire, yardstick, fathoms...
"You get to start 1st grade in two more years" - grandma, mentors ...
Nuh-uh,
the peanut butter and jelly sandwiches and recess sounded pretty good , but my gut said there was going to a lot more Skool and I wanted to hang with mentors and grandma, and shoot my wittle gun, go fishing, and mess with dawgies.
"This gun stuff will get more complicated as time goes on, folks just have to mess with a good thing" - Mentors.
Of Course they were correct...
4 years old -
I got a B&S wire gauge in one hand, a 20 gauge shotgun shell in the other, and nowhere did I see how this shell was a 20 gauge.
I just "knew" there was a "Browning gauge". Had to be.
25ACP "converted" to 6.35mm Browning. Okay, the neat Browning, Colt, and Beretta pistols that shot this cute ctg are sitting there on the table, Mentors cleaning, and talking and ..."how come a Beretta, used a Colt ctg,and was all this measured by a "Browning gauge"?
I did a lot of sitting on Mentors laps with a puzzled look and "I think Young'Un got another question, spit it out boy..."
11mm, 12mm? Are you folks nuts?