@Walkalong &
@GW Staar - that RCBS page is poorly worded.
Side by side, when I set both to bottom out on the shellholder, the headspace is nominally the same for the small base vs. standard FL sizing die. The problem, of course, is that whenever I Small Base size, I want to run it as deep as I can, while when I do a standard FL size, I only want to bump sufficiently to offer 1-2thou chamber clearance. So while the die itself may not actually be any different for headspace, how we use the dies and how they are set up, due to their respective nature, does change. The small base should be set up to touch the shell holder to size the base as deeply as possible, whereas the FL sizer might not be touching so the headspace better matches my rifle chamber.
If I am honest, I have wanted to cut the shoulder and neck out of my small base dies for many years. The fact I want to minimize my base diameter doesn’t also mean I want to minimize my headspace length, so I would love the ability to separate these steps. The only reason I haven’t done so, is the realization that it would likely introduce some opportunity for runout. My only satisfactory solution has been to NOT small base size every time, and to NOT small base size in my batch right before a match where extreme precision is demanded.
Here’s the big difference I have experienced between outside turning and inside reaming:
FIRST: both work to uniform brass necks, but the reloader has to understand how both systems really work so they can use them properly, designing an appropriate process around their gear. Otherwise there’s opportunity to trip yourself.
SECOND: I fervently prefer lathe turning, evidence for why I hold this opinion is detailed below.
NOTE: I could be completely wrong in this, and would love to get better educated, but as a product design engineer, including some tool & die design work, with disposable income to have bought and compared all of these methods over the years, and a 25+ year reloader using this gear, these are my observations for the two options...
Turning: you’re forcing the brass over a mandrel and wedging it througha controlled depth cutter gap. In principle, this is not so different than a Doctor’s knife or wood planer - force material through a fixed gap with a cutter on one side to make it fit, and guess what - it comes out the other side at the size of your gap. If your neck is oversized and loose on the mandrel, you still yield consistent thickness because you’re forcing the neck between the annular gap between the mandrel and the cutter, and the slack between the neck and mandrel would be pushed to the opposite side. If the neck is sized too small for the mandrel, either it gets sized when forced onto the mandrel, or it has to be expanded to fit, so it really can’t be undersized. Since the reloader picks the gap, as long as the brass is thick enough to fill the gap, it will cut to uniform thickness (and the reloader may choose to NOT make a full perimeter cut, depending upon their intent for the brass, cleaning up to 90% cut might be sufficient. A guy can buy their mandrel to match their expanding mandrel, such the resulting brass is ready to be loaded (if a guy didn’t want to anneal and size/expand after turning anyway). Brass chips stay outside of the case, so a guy doesn’t have to worry about sending them down their bore if they don’t all shake out. Pretty hard to screw this up, and you have ultimate control over the thickness.
Reaming: you’re holding the brass in the die, and reaming a hole. You have no control over the brass thickness, at all. The thickness is set by the die Neck ID and reamer OD. To control thickness, a reloader would have to have an entire set of precision ground reamers of varying OD. The neck MUST be expanded to be contacting the ID of the die neck, otherwise it can float and the reamer can cut the necks eccentrically. If you have obloid necks, or eccentric necks of non-uniform thickness, such one side is making contact but not the other, the reamer may not cut concentrically to the neck OD. To mitigate all of this, the reloader needs to mandrel expand to larger than the die ID, ream, anneal, size, and expand again. We can’t control the dimension, so we are forced to settle for exactly the thickness the gear produces, and accept the fact that might mean we cannot uniform thinner necked brass, and might mean we have to cut more off of thicker brass than necessary to create uniformity (debatably reducing brass life by making the neck thinner and more prone to split). All of the brass chips end up inside the case, which doesn’t make me feel great, as sending brass buckshot down my bore at 3,000fps just doesn’t make me feel warm and fuzzy. I am ridiculously stringent about washing and dumping chips after reaming before loading. The real downside I see in reaming, above everything else above, is the fact the brass needs to be oversized. I run fired brass (or at least minimally sized) into the reaming die to ensure it is hard against the wall, which might mean in final FL sizing, I push the shoulder back more and end up with some non-reamed shoulder material pushed up into the neck (or really, the neck pushed down into the non-reamed shoulder mass). So my effort to eliminate a step/doughnut and to uniform necks might actually produce a step/doughnut.
Guys debate back and forth if you can push a step to the outside, or push a doughnut to the inside, whether with gear on the bench or firing in the chamber. It’s irrefutable when you do it for yourself - ream a step into a neck and then run an oversized mandrel, you’ll see a doughnut on the outside. The efficacy of that dimensional shift is highly debatable and nearly impossible to quantify, however. Firing certainly moves everything to the inside, but again, to what efficacy remains in question. Axial disparity in neck thickness is a critical problem - the only satisfactory answer I have found is to turn/ream deeper than I might have expected, and let brass flow where it will.
Like I said, I developed the above opinions by analyzing the results of these processes through the eyes of a reloader, as well as an engineer. If someone can disprove any of it, I’d be happy to find new information to better refine my reloading process.
I like quantitative control, and I like simplified process designs. On a Lean Manufacturing basis, much of the preparation steps to optimize around reaming are not value-add. Turning gives me control over the product dimensions, eliminates multiple process aspects where error can be introduced, and reduces some of the processing steps I have to go through. So I prefer Turning.