He went prone out of exasperation from the lack of accuracy demonstrated prior.
And, just as I did when I ran into that problem with a friend's issued Gen3 22, he shot his usual pistol (A Gen3 19) from the same position and got radically better accuracy.
So I'm holding it against the gun.
He's the kind of friend who can summon a Glock rep, so hopefully he will get some useful input on what's going on.