We also have a 1898 or 1900 rule - but - there must be NO factory ammo available for it - otherwise, no different than any other firearm - firearm license required.
So - will it shoot factory any ammo as in 2 3/4" - then firearm.

As most factory 2 3/4" 16 bore is 2 5/8" there is usually no problem in 2 1/2" chambers. Besides, shooting safely is not necessary - will factory ammo chamber and shoot, is the only aspect that matters.