I'm looking for a body die in 300 H&H, but Redding doesn't make one. How would I go about opening up the neck bore on a standard sizing die? Is this something any machinist could do, or would I need a tool and die guy?

I'd contact Redding.

I was having trouble w/expander ball induced runout with an RCBS sizer. It was not a defective die, just that the case being unsupported when expander pulled thru would tip the case and pull necks off center a few thousandths even after I had polished expander.

Called RCBS and asked if they could open up the neck of my die so that it would size neck to proper size for my rifle and thus not require inside neck expander. They asked for two once fired cases and the die and in a couple of weeks returned my die as requested at no charge. Problem solved; now that's customer service.

Regards,
hm

I asked a similar question of a gunsmith years ago, he told me "No way Jim, those things are harder than woodpecker lips".

I would conatct a die maker too.

jim