Notropis,
Attempting to neck-size with a full-length sizing die sometimes does not work well. This is particularly true with cartridges that have very little taper in the body. What happens is that the case body at the junction of the shoulder comes into contact with the sizing die well before the ram tops out. This actually "sizes" and reduces the diameter at the top of the body a bit, which has the side effect of lengthening the body just a bit. This partially-sized round is now actually too long to fit in the rifle's chamber. The only recourse, then, with the full-length sizing die is to go ahead and screw it down and full-length size the case. By the way, this is actually when the case neck stretches and gains in length - during the final stages of full-length sizing when the shoulder is shoved downward and the case brass extrudes forward into the neck.
If one wants to neck-size only, the best course is to obtain a neck-size die. This die is designed to not contact the case body or shoulder. However, neck-sizing can be done without spending the money for a new die if one is cleaver. As far as I know this is originally my idea as I have not heard or read of anyone else doing it, though I am not particularly well-read. The idea is to use a sizing die actually made for a different cartridge of the same bullet caliber, but with a larger diameter body. This way the case body does not contact the sizing die. The die must also be for a cartridge that is not so much longer than the cartridge to be sized so the neck can reach the neck portion inside the die. One example I can cite that I have used successfully is neck-sizing 7x57 cases in a 7-08 die. The 7-08 case body is larger in diameter at the shoulder than the 7x57 so only the neck of the 7x57 case contacts the die. Also, the 7-08 case is shorter than the 7x57, so there is no problem with the neck of the 7x57 case reaching the neck portion of the 7-08 sizing die. However, care must be taken to properly adjust the die so that only the neck of the case is sized when the ram of the press is fully raised. My 7-08 die had to be backed-off considerably for this purpose so as not to transform my precious 7x57 brass into 7-08s with a bad headstamp.
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