A Full lenght sizer could neck size if you unscrew-it enough. You could do also a partial lenght sizing !.
Unscrew the die until it doesn't touch the case then screw-it enough for your need.
How much, how many turns, good question!.
To see what happen and trim the amount of case that you will resize, you may "smoke" it (blacken) and you will see the progress as you will screw the tool.
I had tried this trick successfully before buying a collet die.
i have done a fair amount of neck sizing with my .223 Handi'; but have to keep an eye on it because it eventually allows the brass to expand to where it can be tougher to chamber or extract. a good alternative - some say - is to do the partial-length re-sizing using a full-length die that is unscrewed a little at a time to see how far out one can go before the (previously-fired) rounds don't want to rechamber. some don't care for it, but the principle makes sense to me. i just don't remember doing too much partial-length sizing, because i haven't lost that much brass to full-length resizing like some claim.
i think that the lubricant i use, Mobil-1 in 5w-30 or heavier, has allowed me to achieve a longer case life through less stretching of the brass. then too, i polish the expander balls in my dies with a 600 grit wet/dry machinists paper to eliminate drag on the ball against the case neck. i lube the inside of the case necks with the Mobil-1 and wash it all off in very hot water with Ajaz Lemon dish soap.
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