Re: give up on the custom 223 wssm now what
Not sure, but sounds like you are not bumping the shoulder back. This is a hot cartridge and it will push some brass.
If this brass has been fired 3-5x it will keep moving forward to the chamber dimensions and eventually grow past the shoulder of the chamber.
Take one of hard to chamber cases, buy a stoney point comparator for measuring the shoulder and mike that case and compare it against a case that will chamber freely. My bet is you will find around .002 difference with the hard to chamber case being moved more forward.
NOTE: If you ever have a barrel chambered have the gunsmith make you a "bump guage" to measure your cases. Cost is about $15-20.
Measure all your hard cases to confirm and then set aside.
Take one hard case, lube and work die down 1/16 turn at a time until you can measure .001 setback from original dimension. Try to chamber. Ideally without firing pin in the bolt, that way you get an accurate feel. You might find you are as far out as .002 needed to chamber if they are really tight now.
As for thick necks, that makes no sense. Most factory cases are around .013-.016 max in thickness. You would normally have at least another .004 in neck diameter for most factory chambers.
If the case length grows too long it will be hard to chamber and feel like a tight neck. Another thing to check.