mine was a revelation (very much like a Savage 63)
Thanks for your post. I now realise that the 320K and 346 models and others all are quite a bit different than the Revelation model I had. The revelation model and the similar Savage/Stevens models had a much shorter reciever than most of the Mossbergs. The only think common to them all is that they are termed hammerless bolt actions. I am still interested in the 320k single shot if I can finde one. Very hard to find right now though. The one I saw at the gunshop was picked up before I could decide on it. I did find one other person on a site listing that he had a (342?) carbine with pull-down front grip that he constantly had pin breaks on. I also found a website that listed the firing pins used. What amazed me is that many of Mossberg's hammerless bolt 22s used diefferent firing pins. Some of the guns had a pin similar to that found in my revelation in that they had a small narrow part at the front for the end that strikes the rim. These were early models and later model pins were a flat blade front for the striker. My guess now is that the pin break problem was not as common as I thought. And it may have less to do with the old narrow pin being weak by design, but that the problem was compounded with possibly week metal in the pins and the pins may have not been fitted right.
So I'm willing to give the 320k a try. I see a 320 boyscout single shot on GunsAmerica and may choose this if I can't find the 320k.