Tom,
If you do try and convert the Ruger 12 gauge, you will either want to make it a big bore rifle or reduce your barrels' diameter rapidly so you don't end up with a very heavy gun. I did do one conversion of a Ruger Red Label but it was a 20 gauge. I converted it to a 45/70 for a guy down in Florida who wanted a red dot scope on it for his bad eyes. He hunts deer and wild hogs sucessfully with it down there in the swamps. Here are a couple of pictures of it. The first two pictures of it were taken during the construction process, (the gun is not finished in them.) Bob