Can't use steel in a gun built in 1873.
As for the effect on humans I admit my grandfather who had eaten shot game all his life (existed on it during the war.) showed some signs - he tended to ramble a bit in his 91st year
The evidence of toxicity is not at all strong, in fact it's very weak and founded on surmise and predjudice - there is no hard evidence of toxicity to humans or waterfowl - it's just people at the wildfowl trust etc being constructive with the little evidence they have scraped together to try and make a story.
I think with the stuff in the news this week they were very selective with the scientific date ; they didn't mention the fact that you had to eat something like a thousand pheasant a year before there was any indication of a rise in the lead levels in your body.