35 Whelan Down to 338/06, Outside Neck Turning Question


Aug 19, 2012
I used new Lapua 30-06 necked up to 338/06 specs. A loaded round's neck is .362" OD, and a fired case neck OD is .372". That's what, .005" clearance per side? How much clearance do I need from the necked down Whelan brass and how will I know if a donut is forming? Haven't got my new Lot of 35 Whelan brass (Remington) to neck down/load for OD measurement. This is the factory Mark V ULWT 338/06 A-Square barrel. Any help as I have never had to neck turn anything I ever necked down (.284 to 6.5; 6mm to .224; .308 to 6mm; .284 to 6mm; .458 to .416, to name a few. Not all were factory barrels either. I'm ignorant, help?
.005" per side neck clearance is way more than enough . the looser this is the more it works the case neck , and allows carbon to build on the outside case neck more easily . you only "need" enough clearance for the neck to expand and release the bullet . but you should allow enough clearance for a dirty chamber ,such as carbon or other things . .002 - .003 total seems to be a desired clearance range . my custom rifles will not drop a bullet into a fired case , the bullet needs pushed into the case by finger . if a bullet drops into a fired case you have more than enough clearance . I've read a touch more clearance could help with a smoother bullet release , aiding accuracy . I can't prove this one way or the other .

a donut is formed at the neck shoulder junction . it's caused by pushing thicker shoulder brass up into the neck . I neck turn the outside of the neck donut . I also ream the inside of the neck to cut out the donut . DO NOT believe the hype of push the donut to the outside with a mandrel or expander button and turn it off . it doesn't work that way , it's not that easy to get rid of .

you will feel the donut when you neck expand with a mandrel . you'll also feel it when seating a bullet . you can measure it with pin gauges , inside the neck . or a micrometer on the OD of the neck .
Thank you sir, I think I have a better grasp of it now. Seems I have to wait not only on my brass but a fired case from the new chambering. "But , Mother, I don't want to wait"...lol
have your gunsmith tell you the chamber neck diameter . this way you will know the correct size , and can plan your case prep accordingly .