Bullets fire from adaptors can hit the broad side of a barn. However, you have to be very lucky to buy a commercial one that is going to have anything near the accuracy of the gun without the adaptor. The reason is because the adaptor comes in and out of the chamber every round and the location isn't repeatable. Just two causes of variation:
Adaptor OD to Bore ID:
Chambers have tolerances and min and a max diameter. In order for a commercially made adaptor to fit any gun the outer diameter of the adaptor has to be made smaller than the minimum diameter of the chamber. That means in most guns the adaptor will be too loose. To fix this problem adaptor manufacturers machine grooves for O-rings that take up the extra gap and keep the adaptor from rattling around in the chamber. However, relying on uniformly compressing an o-ring is not a repeatable way to locate something. So every time the adaptor comes out of the gun to remove the spent case and then is reload into the chamber with a new cartridge it is in a little bit different position. This throws of the point of aim.
Adaptor Bore to OD of Adaptor:
Then you have the variations in the actual bore of the adaptor. No machinist can get the bore of the adaptor exactly inline with the outer diameter of the adaptor. This misalignment causes the point of aim to vary as you spin the adaptor.
Add these two causes of variation up and you get large groups.
There are some companies that specialize in making adaptors custom fit to a gun. This takes lots of hand fitting so these adaptors cost hundreds or thousands of dollars. Generally they are used in very high end shotguns so that the owner can fire various gauge shells in his gun that he spent thousands of dollars having a custom stock fitted to his every wish. Someone that spends tens of thousands on a gun doesn't blink about spending a grand to get subgauge adaptors for his gun. (
www.briley.com has customer subgauge tubes for $695 for one gauge or $1895 for a set of 3 (20, 28, 410))
