I had the same problem when I had mine. Same dies but a reground reamer from a 45-70. Wound up putting them in my lathe and taking a little off the dia. Fire formed and they were fine after that.




JMH

As predicted from a prominent supplier I am getting nowhere with C4HD. I will write up the entire process once I calm down.

That was my plan if C4HD did not stand behind their product. Just seems a shame to have to do this to an extremely high priced die only because C4HD does not agree with the rest of the industry on what the dimensions should be,

Very possibly you dont need to shorten the die. Very likely you only need to push the case farther into the die by just a few thousandths inch.  You can at times achieve that simply by switching the shell holder to one with a thinner top.  If by just switching shell holders does not get you enough more sizing then you can grind the top of the shell holder down a few thousandths and that will have the same effect as shortening the die. Far better to take chances with a shell holder than an expensive die. 
   Another way around this is to remove the decapping assembly  from the die and back the die out of the press about half way.  Now lube your case as normal and then set the case on top of the shell holder in the press ram not in it but on it. Slowly adjust the die down until your sized cases fit your chamber normally.  To do this you need a brass drift punch and a small hammer. This is to eject the case from the die. I do this often and it causes no damage to the case. You only need to do this one time because after the brass is fired in your rifle then your die will work as normal for subsequent re-loads.

All great ideas but I have measured everything and the die sizes correctly if the case is pushed all the way in (ie on the top of the shell holder) and I have tried several different shell holder manufacturers but 0.050" is a long way to go. My plan is to do just that but disappointed that given all of the data C4HD refuses to adapt to the standard that PTG, and Shiloh (how gave up arguing with those people), and several other references call out.

My other thing is to make a die out of aluminum using the PTG chamber finishing reamer for my first sizing before final sizing as above since it tends to wrinkle the cases if going at it to hard or if the case has any imperfections.
Sorry for the rant but now I have to check my 25-25 set to see if that is correct. 
Charles

I've had to cut several dies in the past and just used a carbide bit.

I ended up with a very similar problem ( though not on a necked case ) with my CPA 38-50 and CH4D dies.  The CPA chamber is super tight at the base and I can only get the brass to chamber using the suggested technique of sizing without using the shell holder.

I ended up cutting off part of a 303 British die ( removed the neck and shoulder ) and then took about 0.1" off of the base and with that I can now do the hard part of forming with that die.  Is there any chance there are another (cheaper) die or two that you could modify to get the job done?