Thanks - out of interest I've located an email from the Australian shooter I mentioned, who said as follows. (It was actually an RSIII stock but I think they are essentially identical for current purposes). Obviously this relates to the Barnard P action rather than the G+E round action, but the principles should be similar.
Meredith passed your enquiry on to me. My stock is a Grunig Revolution RS III. I have been shooting it for almost two years with, as you correctly identified, a Barnard P-model action. And you are also correct in that a Barnard will not drop straight into a Revolution stock. The solution is to make an adaptor block that acts as a vee-block for the action and is bolted into the stock to allow easy removal. I have two actions and one stock. The vee-block is bolted to the action via the normal take down screws and then the action/vee-block is bolted into the stock using a small which is already in the stock to allow consistent location of the adaptor block. There is only one modification to the stock that is required and that is to mill a small recess to allow clearance for the front take-down screw.
The idea for an adaptor came from the fact that Grunig sell an adaptor that will allow fitting of a 14-series Anschutz into the stock. In fact, the RS III allows at least 8 different actions to be fitted including a 19- or 20-series Anschutz (these bolt straight in with no modification). You can even fit a magazine action if you so desire! My adaptor is made of stainless steel to give the vee some rigidity. I originally made one of aluminium but I was starting to see signs of distortion in the adaptor due to the recoiling of a 308. Bear in mind, these stocks are designed to shoot 300m, so I assume they thought people would shoot 308s in them as well as the smaller calibres.
I hope this helps. Before I bought the stock I contacted Grunig about fitting the Barnard (which they had never heard of), and whilst they were supportive of my idea to fit a Barnard they were not willing to change the inletting to allow one to fit more easily. I think they are the best stock going around, and provided the longevity is there in terms of the ability of the stock to stand up to the recoil of a 308 I wouldn't have anything else. Certainly, there are no signs of any problems thus far. The only other consideration is that if you do buy one; get the flip up cheek piece as this will allow you to set the cheek piece where it should be for your head and not where it needs to be so you can open the bolt fully.
I've attached some pics of the block for you and if you have any further questions please reply on this email address. Also, I could get a block made and sent over if you think you'll get a stock as it would be easier than trying to reinvent the wheel. Oh, and one final thought. The stock is quite heavy so you will need to consider the profile of any barrel you fit. The heavy PALMA profiles that some manufacturers fit now will take you over the 6.5 kg mark very easily.