On my second attempt to acquire a Steven rifle in 25-20 SS I have been successful.  My first try resulted in a 25-20 WCF which is OK because its a nice rifle.
Shooting this one for the first time today was a rewarding experience as it is wonderfully accurate.  As is my normal fashion with rifles using rare brass I wanted to see how little neck sizing I could get away with and found for this rifle it is zero.
  Dont take me wrong as I couldnt ask for a better situation than that to just re-prime and ready.  The Jamison brass I am using after having been fired has an inside neck diameter that is a perfect fit for a .257" diameter bullet. That is great and should help extend case life greatly but as is also my manner to ask stupid questions that dont need to be answered, why ?
    To me a .257" bullet is on the skinny side for a 25 but as stated I have zero experience loading a 25-20 SS but lots of experience with the 25-20 WCF and several other small case 25's.  Thats OK I can live with ,257" as standard but again why.
    Pondering that question I come up with two possibilities one the chamber neck is very tight or two the Jamison brass is a few .001" larger in diameter in the necks than was the original WRA or the G&H brass. Of those two I am leaning toward the brass.  Please dont try to fix my problem because one does not exist. Everything is perfect and more than I could have hoped for. However if anyone else is using Jamison brass in a 25-20 SS it would be interesting to me to learn about how they fit for you with what bullet diameter. 
Thanks !

When using hard alloy .257 bullets in fixed ammo I get lead in the barrel. .258, no lead. Douglas 10 twist barrel.

Standard reloading practice is if you do not have a slip fit of same bullet in a fired case then you have a problem to fix. I can't speak for Jamison brass as I do not have any. I am currently testing  a new to me model 44 with an excellent factory barrel in 25-20 SS and using .259 bullets.  The bore slugs at .2575 and using Bertram cases that I anealed before I started using them a bullet is a somewhat snug but slip fit in a fired case. I used just 4 cases to develop a load and they are loaded for the 10th time.  I bought a set of Redding FL dies because they are on sale at Grafs. There is very little sizing going on with my chamber and that die set other than the necks. I have a means to neck size  but haven't gone there yet.  It's also very possible that with the low pressures involved that your brass if not anealed is springing back? 

.258s in Jamison brass a tight fit in the chamber. .257s a better fit in the chamber but leaded the bore.

My Stevens 44 action breech seats reasonably soft lead bullets using a plugged case.  Use a piece of 1/4" dowel to make the plugged case.  No more worries about neck diameters or such.

I just pulled the neck expander ball out of my FL die and miked it at .256" spot on.  That expander ball will slip into a fired case neck with slight resistance. Not much but a slight push is required to get it to pass through.
One thing I did not do prior to shooting was to anneal the brass.
I should do that.
I could turn about .001" off the case necks and that may well prove something. As is options are limited. If I turn .002" off the necks I would gain diameter options that could lead to improved accuracy but I doubt it since it shoots so good with .257" bullets.
Turning necks might have a negative effect on case life which is right at the top of the list of importance.  Its a short list, accuracy, case life, simplicity.

You can always neck turn a few cases.

With a .257" bullet loaded in a fired but un-sized case the case neck OD is .280"