Caliber is American, MM is Metric. One type of munitions can be called two different sizes.
A 30-06 in MM is a 7.62x61 round. Also considered a 30 cal.
9mm is cheeper cause it's a smaller round.
The equivalent of 5.65 mm ammunition is .22 caliber ammunition.
buffaloarms.com
There are various types of ammunition. There are also particular guns that use the recently developed 30 mm ammunition. Among them are aircraft guns and armoured vehicle guns.
.30 caliber.
3/4 of an inch is 19 mm.
45 cal. Long Colt (.45LC) rounds like those used in a 45 cal. revolver. Many of this type of Derringer also can fire .410 gauge shotgun shells. Do a search for your specific model and get the manufacturers recommendations for ammunition.
6.8mm / .270
If the barrel has 5.56 printed on it or you have verified with the manufacturer that ALL of their barrels are good for 5.56 ammunition.
7.65 = .32 ACP
ATK Small Caliber Systems is the world's leading producer of high-quality ammunition and links for the military in 5.56 mm, 7.62 mm and .50-caliber rounds.