The dimensions on 4d reamer shows reamer at neck of 0.253 for both the 219 Zipper and Improved.  I see Baco uses Starline 38/55 to form 219 and my Starline 38/55 is just over 0.080.   

I second Jim Carstensen.  He converted a die for me in 7mm Ackley Krag and I just size about a 1/4" of the neck.  The rest of the neck centers the case in the chamber.  I also have had Whidden make me a die in a 6mm-30/30 Improved because I could not find a parent die for Jim to convert.  The die is easily used as a f/l if needed or just size part of the neck most of the time.  I also think Harrell's makes custom dies but have never used them.  The proper die will still not fix the problem of too thin of a neck but should increase accuracy.  You could have Rocky Mountain turn some custom brass if you think the problem warrants better fitting brass but that is not a cheap solution either.  What are you currently using for a sizing die?  Tom

Let me approach it a different way, caution am no expert and most on this forum know more about it than I do.

Wall thickness 30/30 and brass on 38/55 head size is going to be “about” 0.011. variation brand to brand and lot to lot.  Only way to get precise wall thickness is turn all same. Most common issue necking down wall thickness will increase & neck turning needed to bring it back to uniform.

Problem is to get the case to fit your bullet and chamber. Bullet first, cast vey different than jacketed. Most use cast bullets oversize and can’t stand much swaging while being seated. Jacketed bullet can accept a tight case fit not happy with over bore size.  First thing is determine what bullet Diameter you need then prepare the case to suit.

Next is sizing the case neck to suit the ideal bullet diameter. Most factory dies over size then open up with expander dies. Easy to use different size expanders and get it right, overworks the brass most times.

I prefer sizing to fit one step with bushing neck sizing dies, from the outside. Less working brass and precise. Problem is you can get neck sizer bushings in any size near impossible to calculate in advance exactly what size is going to work. Try several pick the right one, change bullet or brass another likely to be optional. Having said all that over sizing and opening with expanders works OK

If cast bullet most times with a neck sized case need to run a too small expander die in to give a slight flare, jacketed not needed.   

Loaded it all has to fit your chamber. Factory ammo always undersized for trouble free function. Expands on firing to seal and springs back allowing easy extraction. High pressure helps seal.  Cast bullet lower pressure often gets blow by & dirty necks. Better fit cures it, single shot no feed issue often extra flare helps seal.

Manuals standards or books will get you close not exact. So many variables bullet case chamber bore and pressure on firing tuning the loaded cartridge the individual rifle only way to get it right.

CH4D my pick for neck sizers or expanders. Have adjusted both myself starting with oversize or undersize.

Boats

Thats true, .25 Krag from 100 + year old .30 Army brass different problem compared to modern 30/40 Brass. Its wall thickness and Diameter at the base.

Every gun and case is a different thing

Boats

Thanks, Tom, you reminded me I have a Vickerman 22 seating die - somewhere.

Fired formed me some brass last night. The neck measured .251-.2515” after fire formed. The bullet is slip fit after formed. I ran it in the neck die half way in the neck like one member  suggested on here to do. Will shoot these and come back with results.