Gary, do you have a set of 400 GNR dies in stock?

bullets, not .401" or larger. That Starline brass is a little thinner (0.006" thick) and provides a little room to chamber. Some .30-30 brass is too thick (some 0.009 -0.011"). It varies a whole bunch, rim thickness too.

I have a nice .400 GNR barrel and I need to get back to it. I only have a .401" bullet sizer which is too big, if only by 0.001". I need a 0.400" and might even go 0.399" if I have one custom made. I use LEE push through sizers to re-configure .410" (yes .41 Magnum) bullets for use in 10mm, .40, .38-40 and yes, .400 GNR. It's dark magic to some.

Dave Manson got all technical on the reamer design and I know he had reasons, it's very well calculated to a very slight taper. I quizzed him about just making a straight 0.416" hole for a chamber and he only smiled. He does that to me a lot. lol The front half of the .30-30 brass would just expand to seal gases, at least to my simple mind. You'd have lots more wiggle room for varied brass thickness from makers and batches.