So here's what I'm seeing....
? STRAIGHT WALLThe Redding
taper crimp die for 40S&W and other straight walled cartridges seems to be precisely machined to the correct diameter, fitting over most of the length of the cartridge brass. In this way their TCD acts as a "bulge buster", by removing any case irregularities (at least in the area of the bullet). In this way it is much like the Lee FCD.
The Redding TCD (below) is a one-piece die, simple to construct as long as you can hold the tolerances. The user simply locks the die into place after the adjustment is made. As long as some lubricant is supplied and the die kept clean, everyone is happy.
The Lee FCD does these same actions in 2 steps. First with a carbide ring to slide over the cartridge case to do the "bulge busting", and a second adjustable crimp setting within the die to adjust the crimp diameter. This is more complicated in construction, but less expensive to manufacture. And, being multi-part, the die can come out of adjustment during use.
But everything is fine with either method as long as the case wall remains straight.
? TAPER WALLProblems enter the picture when you get to 9x19mm Luger (and all subsequent 9mm siblings). The problem is that the Luger has a 0.6? taper (1.2? total) designed into the external wall of the cartridge case. This slight taper messes up everything. There is no way to slide a fixed diameter over the length of the case and have it act upon the entire length of the case. And it is very, very difficult to construct a carbide ring with a tapered ID to do part of this work. Even if you could, you'd need to be certain which way the ring got installed. Tapers simply introduce a lot of trouble.
I was hoping that Redding had somehow figured out how to overcome these issues. However, on close inspection, apparently Redding does it the same way Dillon and Hornady do it. They slide a die with a conical interior over the cartridge. The interior is very slightly tapered, something on the order of 5-7?. This leaves a long, graceful taper crimp, but one that does not contact the entire length of the cartridge. The effect is a long taper crimp affecting about 0.1" of the case, starting at the case mouth and working back toward the head. The Dillon seems to be slightly steeper angle, which makes contact with about half that length. There is no bullet contact as the "through hole" of the die is about 0.370".
The Lee taper crimp die does this same thing, only with a much steeper angle. Since the angle is steeper, much less of the cartridge case mouth is acted upon, meaning that much shorter length of the die interior has to be micro-finished (polished). Steeper angle also means that case length has a greater effect upon the finished taper crimp.
So as to the question: What's the smoothest taper crimp die ? I would have to say the Redding seems to be very, very good. It is constructed from very high quality materials using a very slight internal angle. This results in a very satisfying ("smooth") looking and feeling TC that is
not as affected by case length. However, having more case contact area
it will require some slight amount of lube to be present.
However, if you are currently using a Dillon or Hornady taper crimp die, I cannot say that you will see differences worth the added $40 cost of the Redding. It's better than the Lee TCD, but again, probably not worth the price difference. That is, unless you're looking for more consistent crimp results on mixed length brass.
