What is Shell Length – 2.5″, 2.75″, 3″, 3.5?

Sync explains what is shell length on Remington 870 Forum:

Remember that advertised shell length – 2.5″, 2.75″, 3″, 3.5″ – refers to the unfolded length of the shotshell after it’s been fired. Before firing, a shotshell will almost always be shorter due to the crimped front end. Shotgun chambers are sized to fit the shell in its unfolded state to provide the the payload with a straight path into the barrel, so there’ll be extra room in front of an unfired shell that’s the same length as the chamber.

Shotshell and Chamber

Shotshell and Chamber