Vagabond, I have several cartridge adapters and an insert barrel for use in my drillings and in a .20 gauge Ruger Red Label. Accuracy, I find, can be all over the place.
I use both a .32 SWLong and a .32 H&RMag in my 8 x 57 drilling. The .32 pistol bullets (various weights of XTPs) ride down the bore w/o being engraved by the rifling. Accuracy is acceptable to me for grouse, squirrel, cottontail headshots to 75 feet.
I've got two 9 x 18 Makarov adapters for my 9.3x 74 drilling; one shoots acceptably, the other not so much.
Then there is the .30-30 barrel insert that fits in my .20 ga Ruger. I've been messing with it for 9 years w/o finding the accuracy I need for deer to 100 yards. Additionally, for the .30-30 I have both a .32 SWLong and a .32-20 adapter. Neither of these is practical in the over/under shotgun; it makes more sense to download the .30-30 case and have fewer parts to lose, so to speak. I am saving these last two adapters for use in a single shot .30-30, should I ever have one.
Finally, I have a custom made .22 Hornet barrel insert for the rifle barrel on my 9 x 80 exposed hammer drilling. It shoots well enough for my needs.
The cartridge adapters and the .20 ga x .30-30 insert are all from Ace Dube in Alaska
www.mcace.com The hornet insert was made by a gunsmith in Jackson, Wyoming. Come to think of it, the better of the two Makarov adapters is from a smith in Idaho.
Drilling users in Germany routinely use rifle inserts in one or more of their shotgun barrels to add versatility to their already versatile weapons. As you might imagine, these German inserts are really really high tech. No slip fit, slide in, o-ringed tools there! They are custom fit to the individual gun, indexed so that they return to zero after being removed and inserted, clamp in tightly with various camming devices, tighten up with hex wrenches, etc. In contrast, my .20 g x .30-30 insert has an o ring at its muzzle that probably fits differently each time I open and close the shotgun. The .22 Hornet adapter itself rides the bore; I often wonder what it is doing to my rifling.
The chamber adapters all came slightly over sized in the neck so that I could carefully remove metal and fit the neck to my individual rifles.
There you have it-lots and lots more than you wanted to know, and maybe none of it applicable to your situation. Sorry about that. When it comes to honeybees, drillings, and farm ponds, it can be hard to shut me up. LOL