I have a 284 win so I have the brass here and the lapua necked up was .013 and Peterson was .014. before necking up lapua 6.5-284 was .014 and Peterson 284 was.015. don't have the new lapua 284 brass. just gave what info I have to help decide on reamer that might be best for your situation. neck turning is a pain in rear especially for a hunting cartridge. you are going to want a .340-.341 neck to be safe for all the brass out thereThanks! On grafs site, it said the 284win Peterson brass has a neck thickness of 0.0155-0.016. this is concurrent with what I've been able to find in what people have measured and it seems most people say Lapua is about the same.
So if I necked up from 284win to .30 with either of those brands, maybe I'd be in the .015 range? That's pretty standard so it should be just fine in a regular clearance neck chamber.