Quoted:
Started looking at this a bit because a friend of mine has an old (pre-68) Savage 12ga pump shotgun that wouldn't cycle some S&B 12 pellet 00 buck that he'd purchased.
Shotgun is chambered for 23/4" shells, and that's what the box says they are.
Tried it with some S&B 12 pellet #1 Buck that I had and it worked fine.
Turns out the 00 shells are .220" longer than the #1 Buck shells.
Also made me question whether it would eject a fired hull, but haven't had a chance to test that yet.
So I thought it'd be interesting to measure some unfired shells and see what they come out as.
I measured one example of each shell with a pair of Mitutoyo calipers at 3-5 points depending on how consistent each was and rounded the average measurement to the nearest .005"

S&B 2 3/4" 12 pellet 00 Buck 2.565"
S&B 2 3/4" 12 pellet #1 Buck 2.385"
Win. Super X 2 3/4" 9 pellet 00 Buck 2.320"
Win. Military 2 3/4" 9 pellet 00 Buck 2.345"

Fed 2 3/4" tru-ball slug 2.265"

Aquila Mini-slug 1.440"
S&B 2" 12 pellet #1 Buck 1.860"

And 20ga (all 2 3/4")

Fed tru-ball slug 2.360"
Win #3 Buck 2.365"
Rio 9 pellet #1 Buck 2.465"
Win Rabbit & Squirrel load, 1oz #6 shot 2.300"


Pretty interesting to see a .300" difference in the unfired length of shells which all (supposedly) measure 2 3/4" after being fired. Next time I'm at the range I may shoot one of each and measure the fired hulls to examine that aspect as well.
I also found that my Mossberg 500 (20" barrel, advertised as 7+1 capacity) would hold 6 of the S&B 00 shells with 1 S&B #1 shell, but not 7 S&B 00 shells, which gives me a pretty darn good idea of the length of the shell stack that will fit in the tube (17.775").
If anyone else has other brands/loads I'd be interested to see how they stack up.