No problem to find 16 gauge ammo. The issue is finding pressure data for any commercial ammo, they rarely publish it. 2 1/2” ammo is not lower pressure than 2 3/4”, it’s just shorter. Velocity is also not a reliable indicator of pressure although high velocity in conjunction with heavy shot charges usually results in high pressures the reverse is not necessarily so. Published pressure ratings on specific factory ammo if you can find it is what you are looking for or reliable published reloading data that you can use with components that you can readily source. A basic solution, appropriate to black powder era guns is to load original spec black powder ammo which can be done with minimum tooling or fuss.