First please read my reply a bit better. I said the 6mm would caause a few more problems as it's longer. Perhaps not in overall length but in length from base to shoulder and big green uses sheet metal magazines. Formed to release the round progressively. If the round hits the feed ramp before it's released it'll have a problem. Can be minor or major, depending on tolerances.. The 243 is a direct swap. Now as far as the stock swap, I don't really know if the actions are the same but the barrel channel is not. It is possible to cut out the channel for a larger barrel but the contour you mention is smaller than the Magnum' contour remington put into the 673 stock.. There are several factors involved in putting a barreled action into a stock.. Perhaps a laminate stock from ElkRidge or Wenigs might suit your taste and have a suitable barrel channel.. As I mentioned in the initial reply, try it first it may well feed. If not it should prove an easy fix.. Hope I've clarified my reply for you.