I think the 20,000 refers to a fatigue test of firing 20,000 standard rounds in the gun. Proof testing is a pressure test but also you worry about long term use. They were concerned that joints would fail in regular use. Hence the 20,000 standard service cartridge test. I think Remington subjected a 1100 to a 100,000 round test. Then tore it down and checked for wear and part failure. Friend was given the job to conduction the test. Strapped a gun down to a test jig and spent hours loading it and using a lanyard to pull the trigger. This was done at a Remington facility and he decided to cut a couple trees down that had died at the range. I wish I had that stack of ammo he used.