In a pure sense, it is a measure of the diameter of a bullet, usually given in decimal fractions of an inch (a .357 bullet is about.357 inches thick) In a broader sense, it is the cartridge that a given weapon will fire- such as caliber .38 Special, or caliber 30-06.
About .36 caliber. The 9mm family of calibers normally uses a bullet that is .356 diameter. This is the same as the .380 and nearly the same as the .38 Special, whose "nominal" calibers are quite different from their actual bullet diameter when measured.
Bullet speed can vary hugely between calibers and bullet weights; these need to be known. It is recommended that you do a search on ballistics on the internet and find more specific information.
It depends on the bullet. It can be as little as less than 1 grain, or as much as several grains or more, especially for larger calibers.
If you are talking about a .45 acp The bullet diameter of .451 equates to 11.5mm. If you are talking about a bullet used in rifle calibers (45-70) etc.then the bullet diameter of .457 equates to 11.7mm.
The DE was made in several different calibers. The speed of the bullet (FPS) depends on which cartridge.
The DE was made in 3 different calibers. The speed of the bullet depends on what caliber.
RPA rifles are made in a number of different calibers. There is not one single caliber- sorry.
Caliber is a term used to describe the diameter of a bullet in SAE units. 100 Caliber = 1 inch a 45 caliber bullet has a diameter of .45 inches.
You will have to experiment to find out. Start with available reloading manuals
Some can. I've seen people shoot and kill armadillos with a handgun. Some smaller calibers might not.
18 gauge steel is not bulletproof for rifle rounds but is for some very weak pistol calibers
The short answer is no. Although the two calibers have the same parent case, 7mm Mauser, the 257 uses a .257" diameter bullet and the 6.5 Roberts uses a .264" diameter bullet. They are not interchangeable.