Take another shot at another day at a comparable distance. If he makes it, I would even let him keep the longer one. The problem is, with the current equipment, all shots at those distances are luck. It's nice to be lucky, but marksmanship should be about skill.
Won't it need to be comparable conditions?
Similar height of hide with respect to target. Similar wind conditions (which given the distance and being in a city might be quite difficult to recreate cross-winds), similar humidity etc.
I'm not saying it's impossible, I am wondering how realistic this would be (not the recreated distance of the shot itself, but how likely you could actually recreate conditions immediately after the shot without putting an operator at-risk).
And how far back would you eradicate records? The previous British and Canadian records would seem very suspect to me if they didn't follow similar conditions.
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I'm still going to chalk this up to a lot of skill and whatever luck is.