22 cal are the ones you don't want to dryfire, the pin runs strait into steel, however there some singleshot shotguns that I would not dry fire, like NEF's & H&R's they have a pice of metal that comes forward and covers the firing pin, and they will break, there is probably somthing to not dryfiring a centerfire rifle but it's unclear to me what it is, just better not to and be safe.