Good question. My .35 Remington goes .359" and a lot more do too. That's the correct groove diameter, not .358" (Hatcher's Notebook, page 435). Would a HBWC expand enough to fill the grooves? 3.0 grains of Red Dot is about right for a .303 British, so I'd try 3.0 grains of Bulleye for a start. (Case capacity for a .303 with a 180 gr. bullet is 50 grains water, 44 grains for a .35 Remington with a 200 gr. bullet seated.)
Try popping a slow HBWC into sawdust or wetpack and see if the bullet is expanding to the bottom of the grooves. It's pretty obvious when you look at it.
You could bump the diameter up in a vise or shop press, although the HB might collapse on you. A solid base WC would work, and it doesn't take much pressure to bump them.
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