Consider a "VLD" type neck chamfering tool.  Although I am seating GC bullets for my CBA match rifles I no longer have to deal with the M die.  I think the long taper will work for plain base bullets as well.

lyman  m  die  the best  art

That’s true, rule of thumb case neck wall thickness is about .011. 25/20 WW don’t have exact in front  of me but much thinner.

Also, nothing against Lyman M dies have many and always gave me good results. Today Lyman seems to be size limited and difficult to get the one you need. CH is the actual maker answers the phone gives good advise and keeps large inventory  on hand. Ships same day too.

Boats

Looks like BACo's smallest is 0.262-0.264". That is larger than I want. Don't want to work my brass that much if I can avoid it.

Yes......... I find the BACo website difficult to wonder around on since it was "updated" a couple of years ago.
Thanks.

Glenn

Measured WW 25/20 with the tube mike, case wall thickness  is . 0085 

.273 Neck sizer leaves the necks OD at about .2735 Subtract double the case wall thickness .0017 I calculate about  .256 case  neck internal, that checks out with the internal Mike’s actual case reading.. 

.257 expander opens it up a thousandth & gives .003 pull .260 bullet & WW case combination . You could reduce  the pull with a .258 or .259 expander. Reason for reducing the pull depends  on many variables, bullet size, magazine or single loading rifle mostly . I think .003 pull is about maximum plain base cast.

Can’t say what a fired case neck is none on hand to measure but the sizer doesn’t reduce it much & expander increases it very little. It’s the recipe for long case life and successful fixed loads 

Boats 

True, Gas check bullets  seat like jacketed, almost. At .260 my cast plain base are about .003 oversize, compared to jacketed bullets common dies are made for. They will deform on seating if pressed too hard.

Once you get the sizing and expanding die dimensions right it really easy to run plain base cast.

Boats

The Lee Universal plugs even work on 22 rimfires … use the small point on the little one to expand the case mouths so the heeled base bullets seat properly with no shaving