A related question: is the .30-06 the most widely used cartridge? Popular with civilian hunters, wasn’t it also used in .30 cal Browning machine guns, BAR’s, etc. on the military side?
It probably was the most widely used, for sporting purposes, on the North American continent. I suspect its place may have slipped in recent years, given the popularity of .223 Rem and .308 Win and their military equivalents, and military manufacture will have pretty well ceased around the world given that no new designs have appeared for it in many years and that the WW2 generation of smallarms is slipping out of military usefulness.
Yes it is the same chambering used in all .30 cal machine guns up until the M-60, put into service in 1960 (M-60) the .30-'06Spring. (7.62x69) was shortened to 7.62X51 NATO.
The .30-'06 was also the same as the .30-'03 except for the bullet shape (spitzer, pointed).
One should note that for the .308, .30-06, .30-30, and similar, are all 30 caliber bullets. As Gbro indicates, the shapes and weights (.30-30 comes in 150gr & 170gr as the most popular, but other weights are available) may be different, but the actual bullet diameter is still 30 caliber. The cartridge behind the bullet can be any one of many designs.
The .40S&W & the 10MM are the same diameter, but the cartridge behind it is different (10mm is a bigger load). .357 & .38 are actually the same as well.
Cartridge naming conventions are as inaccurate as anything I’ve ever seen, and are often nothing more than the fancy of the inventor of the cartridge.
How common are rifles with a caliber larger than ~.30 and smaller than the .50 cal.?
There are quite a few rifles in the .33 to .400 range, some in the .408-.416 and scattering above that to the .480.