I have used inserts and chamber adapters in a number of different calibers and gauges. A good source for them is
http://www.mcace.com/.
I have one of their six inch 12 ga. to .45ACP adapters that I drop in the pocket of my hunting vest when carrying a double gun. Surprisingly accurate out to 25 yards.
There are usually a number of different shotgun to pistol caliber adapters available on Ebay. They are from a different outfit; they work as advertised. I own one each for my 20 ga. and 12 ga.
The Old Western Scrounger used to sell them. I bought one for my 577/450 Martini-Henry so that I could shoot 45 Colt cartridges in that big old rifle. Needed to find an appropriate bullet - once I did accuracy was fine. Here's a pic of a group shot at 50 yards - BP load. [img]http://huntingnet.com/forum/

Not MOA by any means but about right for a good adapter.

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