Except of course it would be a real pity to damage, let alone ruin, a hand-built one-hundred-and-fifty-year-old gun for the sake of trying to get a few extra yards of range to miss at. Those vintage boxlocks may be in surplus now, but future generations will thank us for treating them gently.Very comprehensive summary. Thank you.
I often get asked by people if they can use this or that steel shot cartridge out of their old shotgun.
My reply usually lays out the regular recommendations (i.e. no more than 3.2mm from thighter than 1/2 choke). Then I tell them that shotguns that are officially not fit for high velocity steel shot are hardly marketable anyway. So what‘s there to loose? I tell them to shoot whatever they like.. If the gun doesn’t survive, nothing is really lost in terms of value and they can still buy a new shotgun when/if this happens.
Standard steel loads seem to present no problems in vintage guns, other than ballistic inefficiency compared to lead.
It is HP steel that should only be used in suitably proofed guns. It exists to give lead-like performance when a degree of reach is required, because the only way to get steel to strike hard enough at 45 yards is to give it a much stouter kick in the breech.