Embarrasing moment for me a few years ago. Put the open set of 20 gauge barrels on my M21 for a grouse hunt. Pulled the trigger on the first bird up, and got only a "Snap" Finally figured out that I had put the 20 gauge barrels on a 12 gauge frame. Locked up fine, but the firing pin spacing meant that the shells didn't go off. Short grouse hunt, that.