Bear in mind that length (2 3/4, 3, 3 1/2) refers to the unloaded hull, loaded rounds are appreciably shorter. On average a loaded 2 3/4 is 2 5/16 inches and a loaded 3 inch is 2 1/2 to 2 5/8 inches, so you will have to redo your math. There are also variations in loaded ammo length between manufacturers. Spring and follower length factor in as well as they take up space wthin the tube, based on design and make/model of shotgun, so it is difficult to give a generic answer.
I learned something new today! Thanks
I didn't believe you so I went and measured some shells and yep they're all almost half an inch shorter. Now I feel like I've been ripped off all these years!
I know for sure that a 9 3/8" extension on a Benelli SBEII with the factory follower will not hold 6 rounds of 3.5" shells (tried 4 different brands).




















































