Hey! HEY!
For many calibers, we often still call them by their size in inches.
All of these are named by the diameter of the bullet in inches.
eg: .22 means .22 inches.
-rim fire-
.17 hmr
(basically a .22 WMR necked down to .17. rising in popularity as a kind of... more powerful, faster thus flatter trajectory, replacement for longer ranged .22 shooters)
.22 short
(early revolver round, early semi auto round, still used fairly widely for both, today)
.22 long rifle
(still widely used today in carbines and revolvers, as well as down caliber'd variants or kits for 'meaner' looking semi-autos)
.22 WMR
(.22 lr, but magnum. big boy .22)
-center fire-
.223
(the 5.56 before the 5.56 was NATO standardized. very short summary: they basically just put more gunpowder in a .223, and called that 5.56x45. many in the US still use weapons that are made for .223... but you're gonna want to upgrade your barrel to something that can handle the greater gun powder in 5.56 if you are not a fan of your gun exploding in your face when you fire it)
.38
(many variants of this exist, most notably the .38 ACP for semi autos, and the .38 special for revolvers)
(nowadays, its a lower powered variant of the 5.56, but it actually historically preceeded the 5.56. same size, but do not fire a 5.56 out of a .223 gun)
.40, or 'forty cal'
(early attempt at making something meaner than a 9x19mm, led by the FBI, less generally popular today, but was very popular with the FBI for a while)
.45 ACP
(the caliber of the iconic Colt 1911)
.300 blackout
(an 'intermediary' round that is between the NATO 5.56 and 7.62, often used with suppressed weapons)
.357 magnum
(very, very common revolver round. Sig Sauer actually at one point made a .357 sig for use in semi autos... don't think anyone really uses those any more)
30-30, or 'thirty thirty'
(lever action carbine round, been around for over 100 years, like the .357, probably not going away anytime soon, as the lever actions that shoot them have not only remained fairly popular, but also are currently having a bit of a rennaissance with many gun makers in more legally restrictive states offering 'tactical' lever actions with modern housing, collapsable stocks, optics mounts etc)
30.06, or 'thirty ought six'
(basically, a 7.62 NATO that's 12 mm longer, used to be standard in military springfield rifles, also used in the BAR, still used by many hunters today in some kind of rifle)
.338 Lapua Magnum
(specialized sniper rifle round... if you don't count 50 BMG or even larger, anti-materiel rounds, the lapua has the longest recorded, confirmed sniper kill in history... though this may possibly now be incorrect as of the RussoUkraine conflict... point is, its a very capable sniper cartridge, good deal of wealthier US hunters and long range target shooting enthusiasts love it as well)
.410
(for some estoeric reason, this skinnier shotgun round is not referred to with the standard 'gauge' nomenclature)
.44 magnum
(dirty harry's revolver caliber, which will take your head clean off, assuming you do not feel lucky)
45-70
(older, fuck off huge revolver / lever action round)
'50 cal'
(can refer to either the .50 AE, famously used in the Desert Eagle, or the .50 BMG, used in the 'Ma Deuce' M2 Browning Heavy Machine Gun, and the Barret M82 Anti Materiel Rifle)
...
I've almost certainly missed a good number, point being, us American gun nuts... and/or gun nerds... yeah we learned metric, but we still use inches/imperial all the damn time.
We really only call NATO standardized rounds by mm. 9mm, 5.56mm, 7.62mm... and I guess the 6.8 grendel, and newer 6.8x51mm round the Sig Spear / M7 uses... and also I guess we size grenade launcher rounds in mm, but uh, ....civillians generally don't get live grenade launcher rounds in the US.
We had to draw the line somewhere rofl, and apparently it is grenade launchers, hahahah.