Re: what neck for a 6mmx284
My Lapua 6.5-284 has a neck thickness of .015 so I would think you would want at least a .277 (.243+.030 [.015 on ea. side]+.004 clearance) neck to avoid having to turn them. But I'm just guesstimating here not certain.
I don't know about other brass or the reamer difference.
Ignorance can be treated with education, sadly there is no cure for stupidity.