Thanks 180grainer, blackwater and Foxton Gundogs for the heads up, knowing the gun can handle steel shot if it gets modified is very reassuring. I recommended him go see an actual gunsmith rather than just reading forums online. I probed a little bit further and it turns out its a 1950's Winchester Model 12, 12 gauge. Hopefully he can get in to Reliable gun or Italian and talk to someone about getting the choke opened up.

Sharpish I was at Italian today, and they said they didn't have any bismuth unfortunately. Seems like this stuff is rarer than gold, but if it really is that easy to modify a gun to shoot steel I expect that's why.

I believe my friend is heading to Vancouver tomorrow to ask about his gun, I'll give an update once he tells me what the verdict is.