Re: .270Win Chamber too small
I wouldn't touch the shellholder or die base.
If you neck size only, eventually you'll need to FL size anyway!
The reason your brass is hard to chamber after sizing is because it's being lengthened in the die and the shoulder isn't being pushed back, this lengthens the case by squeezing the case walls in and pushing brass forward into the shoulder and neck. Brass is reluctant to take reshaping, it actually springs back inside the die just as it does in the chamber when fired, you need to bump it past the point required, and let it spring back to the desired dimensions. The force required is staggering! The amount of spring back is equally staggering!
The easiest way to 'bump' your shoulder back is to screw the die in until it touches the shellholder, then raise the press handle and screw it in a further 1/8 turn so it cams over on the top stroke of the press handle, make adjustments in small increments (1/32 turns) until you get the desired measurement from the precision mic. Once set, lock everything up, and you'll be set for the life of that rifle.
If the shellholder is only touching the die base when you raise the ram, it will NOT be touching the die base with a case installed, it must be touching the die base when actually sizing a case for it to work correctly!
I have a minium chamber in my 25-06, and I have to have it cam over quite a bit to get .002" below the fireformed case dimensions, this is what the precision mic is for. My chamber is 2.050", I resize back to 2.048" using the precision mic to determine exact headspace, no other tool is required to do this.
Your FL die can quite easily go beyond minimum dimensions, that is what 'oversizing' your brass is, reducing headspace too much.
If you follow the above procedure, you'll get precise fitting of your resized cases to your chamber.
It is not a problem having a minimum chamber, because brass life is extended substantially by not stretching the cases excessively during fireforming, then from fireformed condition to resized condition the brass is being 'worked' the absolute minimum, it is actually a good thing!
Hope this helps you get the desired results.