From the end (where front sight is located) back to where bullet enters barrel.
Which pistol? Most production pistols have a barrel length of 1 to 8 inches.
From mouth of barrel to where it meets the cylinder.
yes
A pistol is a single hand firearm with a short barrel.
No
125mm
Samuel Colt
Ridiculous
@ 125mm
Frame barrel hammer
Caliber is the size of the barrel's inside diameter, in fractions of an inch. A .25 caliber pistol fires a bullet that is about 0.25 inches across. Pistol can range from very small (about 10 caliber) up to very large (75 caliber) and there are hundreds of different cartridge sizes in between.
Beginning of the barrel to end of barrel.