Others may disagree but your Trojan 20 has probably seen more 2 3/4" ammo put through it than you might imagine.
I am not afraid to shoot REASONABLE 2 3/4" ammo in my 20 gauge Trojan.






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I am sure you are right.
What percentage of shotgunners even know that 2 1/2” cartridges even exist?
Not to mention that there are no markings on the barrels to indicate chamber length.
I hadn’t thought about it from that perspective.

I had a 1924 Trojan and the chambers were just short of 2 1/2". Recoil with Win AA target loads was wicked. I wasn't smart enough then to check the case mouth for feathering, indicating the mouth entered the forcing cone.

As often posted, today's nominally 2 3/4" shells often are shorter, and I would suggest some dumpster diving at your club and measuring some empties. It would certainly seem reasonable to pick a shorter than 2 3/4" hull. Please let us know if you find one.

https://photos.smugmug.com/Shotshell.../Shells1-M.jpg

I wonder if I made the right decision, but I had the chambers lengthened to 2 3/4” by someone qualified to do it and haven’t worried since.
I still look for the lightest BP load or Arma from Clays. And also use the Winchester low recoil at 980 FPS ( #AA12FL8)