It's all about the pressure in modern shells, even trap loads can generate high pressure, they don't feel as strong because they are moving a lighter load and possibly slower than heavy field loads. The powder in them burns fast and hard for a shorter distance down the barrel. Modern 2 3/4" shells can hit 11,000psi
A quality damascuss barrel in good condition can handle modern shells. In England you can send you old shotguns to a proof house and have them approved for smokeless powder shells. The smokeless powder shells made for them run from 6,000psi to about 8,000psi and max loads are around 1 1/16oz.
Older damascuss barrels that haven't been cleaned properly after using blackpowder can be sevierly pitted or have rust seep into any faults in the damascuss weld.
Be sure and check the chamber length also, many old damascuss shotguns have short chambers, my LC Smith has short chambers.
Ballistic Products has lots of info for loading shells for older shotguns. As mentioned RST has low pressure shells, and I think Kent has their Game Bore line for older doubles.
It's always a crap shoot firing old guns as you never know how it has been cared for in the past. Even bright shiny bores can be decieving as someone might had the pits honed out of the barrels making the steel in them even thinner. In Europe if that kind of work is done the shotguns are usualy reproofed.
