I am loading for a Remington Sendero in .300 RUM. Factory barrel. I started with a RCBS neck size set and am having problems getting under 1.5” at 100 I decided to get the best dies possible to get that problem out of the equation. I purchased a set of Redding Type S match bushing neck die set with the body die in it. When I ordered the dies I selected a .334 bushing. I think that is where I screwed up, not sure what I measured that day. Once fired cases out of my rifle have less than .001 variation as measured by my Sinclair concentricity gauge. Today I used the dies for the first time. I sized three of the once fired cases and wound up with .004-.007 neck runout!! I thought maybe I got a too small bushing and measured two boxes of factory ammo. Once box measured .338-.339 outside neck diameter. The other box of a different brand measured .336-.338 outside neck diameter. I measured neck thickness of a handful of my cases with my RCBS caliper and came up with .0015-.0018. Using this information, it looks like I might need a little larger bushing like say a .337. Will using a too small bushing cause that much neck runout?