The gun is a 1878 W&C Scott hammer gun with Damascus barrels. The flats on the barrel state a 2.5" length and there is nothing to suggest the gun had the chamber lengthened. The gunsmith stated the barrels minimums are 40 thousandth.
While I am set up to load 2.5" on a Lee Load All, for this gun and a couple other difficult/impossible to replace guns I prefer to use RST or an equivalent. I don't expect to do any high volume shooting with the gun. And yes, I'm probably being far more careful than I need to be.
Being more careful is ok, and preferred over gambling blind.
While the min wall maybe .040, another consideration is min wall at the chamber. While different recommendations exist and it does depend on the steel itself my recommendation is to not lengthen chambers if you would reduce to .095 or less, in a Damascus 16, further more wall thickness is good, if were fluid steel you might go to .090 but I would not.
Thin walls, which you do not appear to have down the barrels, are not in you face, thinner walls created by lengthening the chamber are in your face literally when you shoot.
Much has been written about shooting 2 3/4 in 2 1/2 chambers and the potential increase in pressure. Will it be a increase of 1-200 PSI or more, I can’t say, I don’t know and neither does anyone else for an individual gun of a particular bore size, different forcing cone, the particular Shotshell material etc. Have I shot longer hulls in short chambers, of course I have; Mostly in blissful ignorance.
Today I cut down longer hulls and roll crimp with a harbor freight drill press, cheap but effective.
Most 1oz 2 3/4 loads can be done in a cut down hull roll crimped. Longshot appears to be easier to come by though you may be able to find anything at gun shows if you are will to pay high prices.
As for a load I recommend for 16 2.5 in:
Cheddite hull 2.5 inch roll crimped, 21.0 Longshot, SG16 wad, 1oz lead, Cheddite Primer, 7,600 PSI, 1190 FPS
REM Black hull 2.5 inch roll crimped, 20.0 Longshot, SP16 wad, 1oz lead, Cheddite Primer, 7,500 PSI, 1210 FPS (might need some cheerios in the wad cup to fill space)
Fed 16 hull 2.5 inch roll crimped, 19.0 Longshot, Activ G28 wad (also called SG16 wad), 1oz lead, REM STS Primer, 6450 PSI, 1170 FPS
Make sure you properly crimped, a roll crimped under done turns into a blooper.
Unfortunately with ammo choices and availability reloading is the best way to go.
If you really get into shooting this gun then buy 30 RMC 2.5 inch brass hulls. I have found them addictive, but very expensive. Further they will expand (fire form) to your chambers and if you have a variety of 16s you may find them usable once fired in only a few of your guns. That said, they last forever and I have some reloaded hundreds of times.
