Value will depend on exact configuration, overall condition, finish, box, manual, accessories, serial number, barrel length, etc.. Range could be from 25 USD to multiple thousands of USD.
Not in the current U.S. military. the most common handgun caliber currently is the 9mm.
It denotes a type of 30 caliber ammunition adopted by the Military in 1906.
7.92x57mm, better known as 8mm Mauser. Same caliber as the German military's Mausers.
.44 henry rimfire
The main caliber for the US military is 5.56mm for rifles and 9mm for handguns.
Measure it, compare it to other known cartridges. Most military ammo is not marked wit the caliber/
Impossible to answer without more information.
Russian calibers are typically in millimeter designations and include the following: 7.62x54R, 7.62x39, 5.45x39 rifle, 7.62x25 pistol and other military sizes. There is one that is in a standard English caliber of .220 Russian (there may be more). There are also many "wildcats" that are based off of Russian caliber brass: 6.5mm Grendel, 6mm PPC, .30 walker and I'm sure others.
ABOUT .69 to .75 caliber. These were smoothbore muskets.
With civiian ammo, it is the caliber. Military ammo- year made, and where
Conversion kits are available from Ceiner and auction sites for Colt or military surplus.
As far as self defense handguns go, the .45 ACP is an extremely powerful and effective caliber, with a proven military track-record.