A "caliber handgun" refers to the size or diameter of the bullet that the handgun can shoot. Different handguns can have different calibers, which determine the size of the bullet and its power. Full definition
After detecting the odor of marijuana and observing passengers with open alcoholic beverages, deputies conducted a search and seized a loaded.380 caliber handgun, which they determined had been stolen in 2015 from Clarence. (buffalonews.com)
According to the Bureau, Roof bought a.45 - caliber handgun in April (eight days after he turned 21), despite his admitting to drug possession. (relevantmagazine.com)