I certainly admire your dedication.

We never seem to have received any of the surplus GP-11 here, except in tiny batches sold as collectors ammo.

When I bought my K-11, I got some Lee dies and a bag of new Winchester .284Win brass from my local friendly gun shop. Body diameter at the web is almost identical to Swissicon spec, the neck is a whisker short. The rim of a .284Win is rebated to the same diameter as .308Win. They were run through the Lee FLS die using a .308 shell holder and were basically ready to go.

These ersatz cases worked well, except for the smaller rim not engaging the extractor properly at the point of ejection. Fortunately the Serbian brass arrived on the scene to make things all good.