Case forming: necking down 375 RUM brass

[robbor]

I am playing with the idea of trying to neck down
some 375 rum brass for use in a 300 rum to cut down on neck clearance. I have heard its not a goot idea to do this kind of work in a bushing die and I should use a FL sizer. I am guessing i would have to get a 338 rum sizer to neck down in two steps(338 then 300) and then neck turn to uniform to about a .016 to .017 to come up with .003-4 clearance and see if that helps accuracy any. Initially this would just be a test and I would sort through and take 20 cases to try.
It would be nice to just buy a bushing to do the job. Any recomendations? Right now my necks are showing about .0142 thickness(fed brass) and Im showing a min neck clearance of .010", I measure .009 and added .001 spring back.

 

[abinok]

This is what ill be doing for my 300RUM based wildcat when its time. The added neck thickness is abig asset... especially if youre going to turn anyway. Perfect necks, without the extra expansion every firing. Go for it!

Lee had a limited run of 338RUM dies last year... $25 if you can find them. Makes for a cheap forming die!

 

[Dan B]

I do this for my .243 Winny and 7mm-08 by necking down .308 Winny brass. Works very well!!

 

[abinok]

As do I. Gets you about .002" to .004" more brass to play with depending on make... 308 to 243 that is.