Hi Kaj,
I just got in from the shop. I did some sizing and measuring and this is what I found. First I sized the Magtech brass in my Lee carbide sizer, then I checked it with a .451 cast boolit. There was plenty of neck tension and no problems. Then I dug into my stash of range brass and got out the calipers to measure the actual thickness of the brass at the necks. This is what I found:
Military case .014"
Fed +P case .014"
Fed case .012"
Magtech case .012"
R/P case .012"
Rem/UMC case .011"
Rather than discovering answers, I only found questions. So, I sized one of each and checked them all with the .451 boolit. There was no problem with any, including the thinnest Rem/UMC case. While I do not personally own a set, I have never heard of anyone with a complaint about Hornady dies, presses, or other tools. (I am not and never have been a big fan of their bullets, but the issue there is one of accuracy relative to Sierra's in my rifles). I don't know if your problem is the dies, or if you just got a bad lot of brass. See if your thickness figure agrees with mine. If it does, I think you're going to have to try another set of dies to be sure. Is there another reloader near you so you can borrow his set? I'd say come on over, but it would be a hell of a long drive for you, and some of it would be pretty wet going...