Neck Turning is basically truing up the thickness of the case neck by removing material from the outside of the case. Neck Reaming is attempting to do the same but from the inside of the case. In both cases the intent is to get a consistent tension on the bullet thus a consistent release from the case when fired.
The concept is sound for sub-minute of angle bench rest shooters, but to do so with Military surplus would not produce a benefit.
I would put the effort into Primer pocket reaming and seating as well as consistent bullet seating.
Unless the rifle,scope and shooter combo is capable of verrrrry tiny groups like sub.1/4" or less all the time every time, I highly doubt you you would see 2 cents worth of difference shooting ammo that has been reamed and turned vs. ammo that has not. I know quite a few guys that are into long range accurate shooting and they dont even mess with it mainly the Bench rest comp. shooters that have rifles and glass that cost more than my current car
As Ike and RU stated, there's two reasons for neck turning or reaming. One is matching case necks to a tight target chamber to minimize neck expansion yet allow release of the bullet without driving up pressures.
The second is when forming cases for a cartridge from a different mother case. An example: I make .219 Donaldson Wasp from 30-30 Winchester cases. After pushing back the shoulder the resulting 22 caliber neck is way to thick and needs to be reamed or turned down to allow the bullet to release and not bind in the chamber. Also, when forming cases and necking down like that you may or may not get a little "ring" at the base of the neck. Reaming will eliminate the ring and prevent a bulge at that point when one seats bullets to that depth.
If a case and cartridge are made to the standard of the rifle, i.e. all military and commercial cartridges (not wildcats or customs chambered or formed from something else) it is a lot of work to thin a neck that doesn't need thinning.
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