Re: .224 weatherby necked to 6mm ?
While there are lots of chamberings that match the performance none are belted except wildcats. All the magnums are far more overbore than what you describe. Gee, even a 243 Win is more overbore.
I made a quick model in QuickDesign and QuickLoad.
~30 grains of powder should propel a 95grns Sierra Matchking at an easy 2850fps. The efficiency seems very good.
I am not a fan of belted cases but since you are, if you want an inexpensive to load belted wildcat, go for it..
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