Yes, you’re right, I think I’ve mixed that up with the mention that Sirius had a gun when he ”killed” Pettigrew. There doesn’t seem to be any mention that guns wouldn’t work on wizards, other than maybe Hagrid’s lack of fear of Vernon Dursley’s gun when he got to Harry.
I think there’s a bit in the first book where Harry says his parents were shot, and Hagrid laughs and says no muggle gun could have killed them.
But like, why not? It’s never explained. I’m sure if they survived being shot, magic medicine would sort them out pretty quickly. But there’s no reassign to think a gun couldn’t kill them. Wizards struggle to react fast enough to block spell s most of the time, and bullets seem to move faster than that.
I think the hardest part would be successfully ambushing Voldy, but no reason to think a gun wouldn’t fuck him up if you can hit him.
To be fair, I think that’s to check if their reflexes include unknowingly using magic to protect themselves. That should only work if they’ve perceived the danger, like putting out your hands to stop a fall.
Shooting someone would (if they’re being properly ambushed) usually result in them being shot before they realise it, so it wouldn’t matter if they’re magic.
But if they do realise they’re being shot at, then why defence magic at all? Just use your innate magical reflexes to avoid death!? Why the need to learn how to block spells?
So either guns would work, or they wouldn’t and it’s not explained at all.
Also Hermione slaps Malloy at one point. If his innate defensive magic reflexes didn’t kick in then, no reason to think they would if he was hit with both barrels from a shotgun to the chest.