When using a rifled undersized insert....like a rifle or pistol caliber in a shotgun or a .22 from an adaptor in say a .30-06......you need to have some way to make sure the adaptor goes in the same position each time.
There is a real chance that the outside and the bore are not absolutely cented and parallel. Thus if you have no way of knowing you are putting the thing in the same way then you have no idea where the actual bore is pointing.
As most do not have any means of using the guns ejector to eject the sub caliber cartridge, only the adaptor itself, there is no need to worry about lining the ejector up with anything.
Mark the rim of the adaptor with a felt tip or some such to begin with on the bottom so you can see it as you load the thing in the chamber.
Pick a position, 6 o'clock is easy on most single shot break action shotguns as the ejector is there and a good reference point. Shoot several shots with the adaptor in that position and using the same sight picture and point of aim to give you an idea of how big a group you will be dealing with.
If that group is not where your sights are, experiment. Turn your reference dot on the base of the adaptor to say the 3 oclock position or 12 or 9 and see if that moves your shot group closer to the point of aim you use for slug shooting with or centering your pattern or whatever normal use of the shotgun you have.
Basically play with turning the device until you get something you can live with. At that point make a permanent mark on the adaptor base directly centered over the ejector so you have an easy permanent reference point for loading.
With out a reference point, every time you load a shell and adaptor into your shotgun it is likely as not pointed in a slightly different direction. Failing to develop such a system is the major reason folks complain of inaccuracy in these systems. They will not ever compare to a goof rifle in said caliber, but they can, or at least might, be usable if you do your part.
-kBob
