Years ago I traded for a rifle in .22-250 Rem. when it was a wildcat cartridge. It was easily formed by necking down .250 Savage cases, but in addition to being quite expensive, they were difficult to come by. At the time, match-grade, military-surplus .30-06 cases were only slightly more expensive than dirt, and the quality rivaled that of the best commercial cases available. So the previous owner of the rifle used a set of RCBS dies to form .22-250 cases from .30-06 brass. He included the dies with the rifle, so I did the same.
The wall of a cartridge case is thickest at its base and gradually tapers from there on out to its mouth, and the neck wall of the .22-250 case formed from the longer .30-06 case ends up being thick enough to increase outside neck diameter of a loaded round beyond that of the chamber neck diameter of a rifle.
Included with the form die set was a reamer die and a reamer to be used in thinning the neck wall of a case for a reduction in neck diameter. After reaming, a trip through a full-length resizing die readied a case for loading. It was a laborious process, but in those days it was a cheap way for someone who had more time than money to come up with a supply of .22-250 cases.
RCBS still offers reamer dies and reamers for a number of wildcat cartridges requiring a large amount of brass to be removed from the neck of a case during the forming process. One that comes to mind is the .219 Donaldson Wasp, which was originally formed by shortening the .219 Zipper case and now would be formed from .30-30 Win. Another is a drastically shortened version of the .308 Win. called the .308x1.5-inch Barnes.
Regardless of whether a case is formed at home or commercially produced in a huge factory, wall thickness can vary from one side of the neck to the other. Some brands of are worse than others. As the variation increases so does the runout of a seated bullet, and we all know of its potential affect on accuracy.
While reaming is a good way to thin the case neck wall of a case, it cannot improve thickness uniformity. When a reamer is pushed through the neck of a case, it follows the path of least resistance and in doing so removes the same amount of material over the entire inner surface of the neck. If the neck wall is thicker on one side than on the other to begin with, it will remain so after being reamed even though it has been made thinner.
In addition to reducing neck wall thickness, outside turning will make the wall the exact same thickness all the way around. This is accomplished by forcing it between a snug-fitting pilot on the inside and a steel cutting blade on its outside. Distance between the cutter and the pilot is adjusted for the desired neck wall thickness.
There are two types of outside neck turners. The first one I bought many years ago consisted of a tool designed to be attached to the cutter shaft of a hand-cranked, lathe-type case neck trimmer. It came with pilots of various diameters for cartridges ranging from .224 to .375. Since I also used it for case trimming, I had a set of pilots for that as well as another set of longer pilots for outside turning. Made by Forster-Appelt (now called Forster Products), the exact same tool is still available.
Lyman and RCBS also offer the same type of attachment for their case neck trimmers. Converters that allow a battery-powered screwdriver to replace elbow grease are available.
The second type consists of a cartridge holder and a turning tool. I bought one ages ago from barrel maker Paul Marquart, and during decades of use and thousands of cases its cutter has required resharpening only once. This type is available from Sinclair International, Hornady, Stiller Precision, Forster, K&M, Precision Reloading, PMA and others. The Sinclair battery-powered driver works with all of them.
Most benchrest shooters prefer this type of tool because it is a bit more precise, but they don't use very many cases. A varmint shooter who has hundreds of cases to process will likely be happier with a lathe-type tool powered by batteries, although the other type with a powered driver is pretty darned fast.
Whether or not great uniformity in neck wall thickness is important depends on the rifle and what it is used for. The undersized chamber necks in rifles used by benchrest shooters require it. When preparing a case for that type of rifle, its neck wall is thinned enough to reduce outside neck diameter of a loaded round just enough smaller than chamber neck diameter to allow the case to expand outward just enough to release its grip on the bullet. Doing so precisely aligns the bullet as closely as possible with the bore of the barrel. Allowing the case to expand outward a minimum amount during firing also increases its useful life.
The jury is still out on the benefit of uniforming neck wall thickness for other types of rifles with SAAMI-dimension chambers. It is certainly wasted effort on a lever-action .30-30 capable of no better than four-inch groups at 100 yards.
I think it mostly depends on the level of accuracy a rifle is capable of and the quality of the cases to begin with. When I first added a Cooper Model 22 in 6.5-284 Norma to my battery, I formed cases by necking down .284 brass made by Winchester. When later trying cases made by Lapua, I noticed an immediate improvement in accuracy with the same loads. The difference was not enough to cause me to miss a deer at 300 yards, but group size went from larger than three inches at that distance to smaller than two inches.
As I discovered, bullet runout with Lapua cases was close to zero versus .008 inch for the Winchester cases. Outside turning the necks of the Winchester cases just enough to reduce their thick side to the same thickness as their thin side reduced group size. Accuracy is still not as good as with Lapua cases but with that particular rifle, the improvement is enough to make neck turning worthwhile.