I don't think one hole accuracy was what these people intended for these adapters. The are more for dual use of a pistol, rifle or shotgun. With the ..375JDJ it is a little over kill for grouse or rabbit and damned expensive to shoot small game with. When I carry the 375JDJ for hunting elk or whitetail I keep the adapter in my side pocket of my pants if I come on some grouse, squirrel or a rabbit. I keep a round in the adapter with a small piece of tape over it so I can just drop it in the barrel, pull the tape and fire. After getting the adapter set in the barrel after about a box of shells where it shot the same ok group for small game, I marked it so it would always be the same when inserting it. With out changing my scope setting, I can shoot a soda can at 20 steps, using the reticle where it goes from thin to large at the bottom and holding that dead on.
The one for the 222 Rem Mag was touch and go until after about the tenth brand of ammo I found one that shot well.
Mike