The huge numbers of rounds through specific guns are usually not from average private individuals .
Typically rental guns at shooting ranges . They usually require that rental guns only be used with factory ammo purchased from them , so it would be fairly easy for them to track .
Or also LE / Security agencies that used Dedicated Training Guns for training . Again the number of rounds issued and used during a particular session is known , and fairly easy to track .
FWIW , back when Glocks were first introduced , there was widespread speculation as to their long term durability . In conversation with my local indoor range , he reached into the " rental rack" , and handed me a ( obviously first gen , then the only gen ) G17 that had fired over 100K rounds , with zero parts breakage .
At the time , GlockUSA was loaning certain rental ranges pistols free of charge , with the agreement to keep track of round count and problems , and when Glock eventually took them back , would analyze wear patterns .
Glock got high round count beta testing on somebody elses dime. The ranges charged their usual rental fees ( scratching my memory and guessing inflation , but $5-$10 per rental session , but ammo @ usual markup ) . And And lots of shooters got to try out a then innovative pistol that was subject to great interest in the shooting community . Win - Win- Win .