You are correct that with FL resizing dies with the size button down by the dacapping pin that the neck MAY expand asymmetrically and throw the neck off the cartridge axis, thus degrading accuracy. Sadly, the Redding Type S bushing dies do the same thing, even though they are supposed to solve this problem. The Forster dies do solve this problem, but you don't necessarily have to buy one, and might be able to modify the existing die to work. IF you can spin the expanding button up the threaded decapping rod and get it just below the neck of the size die, you can create a die where the neck is resized while it is STILL in the neck portion of the size die, and then the neck will be concentric with the case axis.
Be aware that is requires some fine tuning, and you will have to get the size button just right below the neck by about 20 mils so that the neck can go over the size ring on the way up and THEN hit the die. If the button is too high, the case won't go into the neck at all, so don't force it!
This is my 338 Ultramag FL resize die, altered to have the expander button up near the neck of the case. I decap the cases in a separate step.
