I don't know of anyone that is currently producing a 300 H&H except for some of the British houses. Could your gunsmith have meant to say 400 H&H Mag? It's a new offering from Holland and Holland, but word is that it isn't going to be propriatary, and some drawings and specs have already been released. Boy, it would sure be a neat round to have in the M77 Africa model or a No.1 H.