The Mine (“min-ee”) shell was developed in Germany during WWII for small caliber cannon in order to maximize the destructive effect against aircraft targets, specifically bombers. 
Designers utilized high-grade steel in a deep drawn manufacturing process to create a thin wall projectile strong enough to withstand the stress of being fired from a gun. The result was a much larger cavity for the explosive payload. In a 20mm shell this meant 17gm HE filler as opposed to 3.7gm for the same shell using existing cast or forging techniques. This also resulted in a lighter projectile which improved ballistic performance. Today, this process is used almost universally for all small caliber cannon types. 
The projectile was marked typically with an "M".

The round at left is a Miné projectile, at right, a forged round.