As a versatile hunting dog owner, I always wanted a drilling. I checked the Internet frequently but all the three-barrel guns I found were too expensive for a country vet. I was at an outdoor show in Selma AL a few years ago and mentioned to a gun man there that I wanted a drilling. He replied "I know a guy that has several and he doesn't know what he's got." (My kind of man!)
Long story short, my wife and I went over to the fellows house (Never take your wife with you to buy a gun!) and I was able to buy a very old 16ga x 16 ga x 8 mm drilling. It was in good shape but had been carried a lot. Made in 1939 in Suhl Germany, my gunsmith "slugged" the barrel to make sure it was a .318 caliber (old drillings can vary in exact caliber, so you want to be sure before you cram a round in that rifle barrel.)
I have loved this gun! With those Drahthaars, I can go down in the swamp and hunt anything that moves. The chokes are both full which limits me on woodcock and quail, but for waterfowl, it is fine. Non-toxic loads for 16ga's are hard to find, mostly bismuth but it has been a effective marsh gun. The rifle barrel is very accurate. It has a button on the tang that elevates the rifle sight when you choose the 8mm barrel.
Keep your eye out for your own drilling. Try to find a widow that has one in the back closet. You won't regret owning one!