How long is the barrel on the Model 12? It's seldom you hear of a bad shooting Martini .22 so it would be a good choice for silhouette. Might be awkward with a scope mount.
You need to be a little more clear on the second gun. There is a large Martini--either Enfield Martin or Martini Henry--conversion for the miniature rifle clubs. These were down by Bonehill , Greener and others. Then there is a Greener Martini. I think you mean the conversion done by Greener, is that correct?.
Can you post us some pictures.
We don't do Mausers here, but you can post over onthe Masuer boards for more info the Mauser .22