I wouldnt hesitate one moment to shoot steel through a modern (Post 1909) barrel AS LONG AS,, The choke is more open than full, and Gun is confirmed to be in relativly good shape.
The standard 12 guage load back in 1909 was a 3 3/4 dram load shooting 1 1/4 ounce of shot.. That hasnt changed at all...
so,, what needs to be considered is the PRESSURE that the FASTER velocity steel loads develop.
It really isnt the steel "SHOT" but rather the pressure increase needed to increase the velocity so that steel load performs well...
Even taking THAT into consideration,, It really only applies with older guns that have double barrels, or guns that rely on Gas or Pressure to operate their actions... Many older SXS's and OU's the barrels are only soft soldered together. the semi autos were designed for that 3 3/4 dram load, so it will operate and not damage the actions parts.. the Modern steel loads,,like I said have increased pressure and velocities for the steel to function efficiently..
A gun like a wingmaster, with single barrel, in good shape, whos action just carries 1 round into the chamber and fires the shot through a tube that meets standards,, and has a choke more open than Full,, is perfectly safe,, and the steel shot WONT damage the barrel.
Just look at the FPS of a modern steel load compared to a lead load... the steel may travel as fast as 1550 fps,compared to a lead load that travels at around 1200.. The pressure increase is significant,, but its NOT the steel SHOT that MAY do the damage.
I have 2 older 870 wingmasters ,,both in 12 guage,, I also have a 1950 ish Winchester Model 12,,that have shot 100's of steel load through them.. One is a fixed choke modified barrel,, the other 2 have screw ins... There is NO,,,, NONE,,, evidence of barrel scoring from the steel shot......NONE! All those guns have confirmed 2 3/4 inch chambers.. something else to consider on older vintage guns..
If you look at the companies that sell "special" lead ammo for vintage guns,, they reduce the PRESSURES of those lead loads so as not to Damage some of the very early vintage guns.. They arnt concerned of safty of the loadbecause the older guns still met pressure requirements of the day... what they are concerned about is the age and the possibilty of soft solder used in many of the older SXS double barreled guns.
Also, the OLD WIFES TALE of a Damascus barreled gun being unsafe to shoot with modern smokeless powder is also crazy.Many people have been told not to shoot those fine old guns, but to just relegate then to hang on the wall.. If the gun is in good condition,, those are very fine guns are a joy to carry in the field, decked out with those cool looking exposed hammers,and beautifully patterened twist steel barrels. Ya just gotta do the home work,and load your own lead ammo for it, to keep the pressures safe..