I have done numerous 6 mm wildcats over the years mainly for 500 Fly, IBS 600 and 1000 yard matches. The occasional Fclass match as well. There are only two reasons to do wildcats, gain accuracy or gain speed. A third if brass flow is a consideration with the parent case. Where you are headed has been done to death for various reasons. The International died because it was neither fast enough nor accurate enough or gave any barrel life advantage. The International has two versions, one based off the 22/250 and used for 300 meter positional shooting then another based off the the 308 x 1 1/2 using the long defunct 308 small rifle primed 308 match cases made by Remington. It was also used in positional shooting and some benchrest though it was a short lived benchrest life but did eventually lead to the birth of the 6 BR Remington, not the longer Norma. 7mm BR Remington came along around the same time for pistol silhouette. The 308 x 1 1/2 was about for years winning many Varmint For Score matches in the US so that case has a lot of history and has been wildcatted down to 224 later to become the 22 BR and as a 6.5, again for pistol silhouette. Cartridges like the XC and others came about through High Power in the US. Each shooter had their own version, own proprietary cases and reamers and ridiculous cost and little gain for Joe average in a bolt gun.
The one thing you have to consider in this country is powder when looking at these mid capacity 6 mils. 2208 is too fast and is faster now with its new formula, 2209 is still too slow. Re15, 16 are somewhat better but unless you have a lot of it hoarded like myself, shipments are few and far between but NIOA is getting some in this year. The reason 6x47 Lap is just about defunct in this country is specifically because it was betwixt and between in the powder choices. Compress 2209 still not enough speed and accuracy suffered, low volume of 2208 high pressure still no speed or accuracy where a stock 6BR was just slightly slower but far more accurate using that same powder. So 6 BR becomes more popular as 2208 is perfect for bore size at that capacity. Lots of Fly shoots are won and records set with 6BR or some derivative of it. 6x47 won nothing but burnt out lots of throats early. So with the capacity around 6x47 Lapua, the case is not an issue, powder choice is. I did a a barrel right at the start of the 6x47 Lap fad. Took a very competitive 6 BR barrel and long chambered it to 6x47, turned it into an uncompetitive barrel with a gain of only 80 fps, short shifted back to 6BR and won heaps with it. So I have been down this road.
So with powder choices being so few right now the thing I would consider is a cartridge volume at 100 load density of 36 grains of 2208 and 45 grains of 2209 in 6 mm to make it worth the effort in both terms of speed and accuracy. So two cartridges come to mind, the 6 GT and the 6 SLR. I did a cartridge around 10 years ago where I wanted the speed of the Dasher but the throat life of the 6 BR. What I ended up with is virtually identical to the GT except for the 35 degree shoulder. Perfect capacity for 6mm and Re15/2208 powder and with no stress runs a 107 grain pill to 2950 fps. Alpha makes brass for the GT and Lapua is about to. I use around 35 grains of either 2208 or Re15.
The SLR is what the 243 should have been with slightly less capacity, 30 degree shoulder with a much longer neck and perfect case volume for 2209 and 6mm bore while giving a longer throat life than the 243 ever would. The beauty of the SLR is that 243 and 308 brass is everywhere and cheap. Velocity in the SLR with 107 grain pills is 3100+ fps using 2209, that being in 30 inch barrels. Another advantage of the SLR is that the brass is formed in one pass and requires no fire forming. Just load and shoot.
After years of doing wildcats and making reamers to suit, I have learnt that while case A makes X speed and shoots tiny groups, modifying case A does not necessarily lead to better accuracy or speed unless the change gets you to the place where the perfect powder or the perfect volume for the powder already in use is found. One example is the old 243. I recently put together a stock 243 for a dedicated thermal rig. Cases are cheap or made from 308 which is even cheaper, but with my stash of Reloader 26 I gain over a 150 fps with the 100 grain Speers compared to any other powder I can get from the local shop. That flattens the trajectory more than I expected over my 6/223 at 2640 fps compared to 3100 fps. Looking at initial 243 data put me off going away from the 6/223 until I saw the Speer print.
https://youtu.be/fzJ7FUWTDVohttps://youtu.be/x8IAy898X6Q?si=YCcth3OmWatvHSTA
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