What does the .50CAL CAL in the bullet represent?

Represents caliber (Caliber?).

Caliber is often expressed in millimeters or inches, such as 7.62mm, 0.45 inches. In English, 0 and inches are often omitted and written as .45 caliber or .45 caliber, pronounced as .45 or .45 caliber.

One inch=25.4mm

0.30in=7.62mm, 0.357in=9mm, 0.38in=9.65mm, 0.45in=11.43mm, 0.50in=12.7mm;

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0.223in=5.56mm (actual=5.66mm).

Extended information:

Early automatic rifles used the "standard rifle ammunition" at that time, which was powerful, but difficult to control during continuous shooting and relatively bulky.

Before and after World War II, some traditional intermediate-power bullets were developed. Although they initially met the needs of continuous rounds, they had many shortcomings. For example, single-shot bullets are still too heavy and expensive, and their trajectory is not straight enough within the effective range, so they are not accepted by countries around the world.

Since the 1960s, small-caliber rifles have begun to become the trend of world army development. From the 5.56 mm of the US military to the 5.45 mm of the former Soviet army, the caliber of the rifles of major military powers has dropped from 7.62 mm to 6. millimeters below and continues to this day.

Baidu Encyclopedia - Caliber

People's Daily Online - Adjustments to the US military's small arms? The caliber of rifles has become larger to penetrate body armor