Well, I guess what I'm thinking is that with a micrometer die I can dial the ogive to lands distance precisiously for each bullet so pressures and speeds may be more consistant. I'm not sure about what an s bushing die may ad for me over a regular sizing die. Could yal explain the diffrence between the 2 sizing dies. Thank's, Brad
Standard full length sizing dies have neck diameters small enoug to size fired case necks down enough so their expander ball coming up through the smaller case mouth will enlarge it enough to hold bullets very firmly. But that typically bends the case necks a little bit; especially if they're a lot thinner on one side than the other. The case mouth is now small enough so all bullets for that caliber will be held tightly regardless of how thick the case neck wall is.
A full length bushing die's one that uses an interchangeable bushing it the top of the die that only sizes the neck. The rest of the die below the bushing sizes the fired case body diameters down a bit and also sets the fired case shoulder back a little. The bushing only swages the fired case neck down just enough to hold the bullet firmly. Bushings come in different diameters for a given caliber so you can use the one that's best for your shooting application. It's best to not use an expander button with these dies but instead use a bushing whose size is 2 or 3 thousandths smaller than a loaded round's neck diameter.
The following link to Redding's web site shows how a bushing die's made and details about them.
Type S Bushing Dies | Redding Reloading Equipment: reloading equipment for rifles, handguns, pistols, revolvers and SAECO bullet casting equipment
The picture has the bushing gold colored. The full bushing die's the best one to use for accuracy; it full length sizes your cases and makes their necks very straight. RCBS also makes these dies. Either is better for best accuracy results and are popular with competitive shooters who know better than to try any die that sizes only the neck because full length sizing dies center the case neck more precicely on the case shoulder. If a sized bottleneck case neck ain't well centered on the case shoulder, there's no way it'll be well centered in the chamber neck.
There's no way one can seat bullets in sized cases such that their jump to the rifling distance is held to less than a .001" spread. The reason's two fold.
First, the shape of the front part of the bullet from its tip back to full bullet diameter varies a little bit. If one measures (with the right tool) the distance from a point near the tip of a 25 caliber bullet that's about 1/10th inch in diameter (about where the bullet seater touches the bullet) back to where the bullet's about .250" in diameter will first contact the rifling, they'll see a few thousandths spread. While the pointing die forming the bullet is the same shape for each bullet, the jacket and core vary enough in composition such that they are not all exactly the same shape.
Second, when bottleneck cases are full length sized and their shoulder's set back a tiny bit, after the sized case comes out of the die, the distance from the case head to a reference point on the case shoulder will vary by a few thousandths of an inch. How much depends on how much lube was used on the case and how much the press springs from pressure.
As the .25-06 round's shoulder is hard against the chamber shoulder when the round fires, the space between the bolt face and case head will vary at least as much as the spread in head to shoulder distance across a batch of sized cases. And the firing pin's impact will set the case shoulder back 1 or 2 thousandths from the force of the firing pin spring driving the case hard into the chamber shoulder.
With the bullet's tip distance from seater contact point to the case head may well have been held to tight tolerances when seating the bullet, the actual round's distance from shoulder to the rifling contact point on the bullet will vary several thousandths of an inch. The only way to keep the distance repeatable from shot to shot is seat the bullets out far enough so they jam into the rifling and set back a few thousandths in the case neck when the round's chambered.
All's not lost accuracy wise if there's several thousandths jump the bullet makes to the rifling. In match conditioned M1 and M14 service rifles shooting commercial .308 Win. match ammo, when their barrels are 2/3rds worn out, the bullets jump about 1/10th inch to the rifling; with a new barrel, bullets jumped only about 10/1000ths inch to the rifling. These well built semiauto rifles would shoot 2/3 MOA (4 inches) at 600 yards with a new barrel and maintain that accuracy level until about 3/4ths worn out around 3000 rounds. At 100 yards, they would shoot under 1/3 MOA. Bolt action rifles are no different and the best of them can shoot bullets jumping 1/10th inch to the rifling more accurate that those match grade service rifles.
There's other things to get right and exact, so I wouldn't loose any sleep over having a few thousandths spread in how far your bullets have to leap from their resting place to where they start getting engraved by the rifling. It's a non issue unless your rifle's capable of shooting no worse than 1/2 MOA (3 inches) at 600 yards. Then you should soft seat your bullets in light neck tension and single load them one at a time.
