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Normmatzen
08-25-2011, 03:47 AM
Does anyone have documented data on the accuracy of turn vs no-turn 6BR/6BRX cases?

Charles E
08-25-2011, 09:33 AM
I don't know what you man by "documented," but it is all anecdotal. From any one individual, the sample size is too small. As with almost everything in Benchrest, there have been no controlled studies with statistically significant data collected and plotted.

On the surface, your question makes sense, but given our sport -- what, 3-4 thousand total participants, including point blank group & score, and long range, it's almost as bad as a "what's best?" question.

Maybe a bit of history would help, though. In the bad old days, case wall variation could be quite large. Guys would chamber a round, and if they extracted it, there would sometimes be rifling marks on one side of the bullet only. Humm, what to do? Turned out, of course, that if you turned the case necks, wall thinness was even, so the bullet was more closely aligned to the bore center when chambered. (The rest of the case, and the chamber itself, still had some influence.)

OK, with some of the brass we have today, case wall thickness is quite well controlled from the manufacturer. Sadly, it isn't as simple as Manufacturer A versus Manufacturer B for all cases; the particular case still needs to be considered. But for .220 Russian, .308, and 6mmBR, Lapua does a pretty good job.

So here's one man's bottom line: It you have a tight chamber (not SAAMI, but cut with a reamer designed to fit a particular brand/lot of cases), and historically, neck wall variation from a manufacturer runs .002 or less, you don't need to turn necks, save perhaps for point-blank BR, where even .005 inch shot dispersement can matter *significantly.*

Alternatively, you can buy cheaper but equally tough brass -- such as Winchester -- cull the cases that are too far outside the tolerance you set, and be at roughly the same place.

When all is said and done, the surety of having equal case neck wall thickness is just simpler to achieve by turning the necks. To get the same effect by culling cases would involve a lot of measuring, and a somewhat higher brass loss due only to culling.

Bob Kingsbury
08-25-2011, 11:08 AM
Add a crooked bushing to nicely turned and uniform necks, and your runout can exceed what the runout is in no turn necks. There are
lots of crooked bushings out there.

Charles E
08-25-2011, 04:14 PM
It's a small point Bob, but a bum bushing will affect either a no-turn or a turned neck -- it's a die problem.

skeetlee
08-25-2011, 06:12 PM
It really is amazing just how bad some neck bushings really are. You no i never gave any of this much thought until i had a conversation with a good friend of mine. What i found after hanging up the phone was quite disappointing. Im not talking about just the cheap steel bushings either. Always check your bushings!!
Charles did give a good point of view i think. I have heard several of the top 600 to 1000 yard shooters say, they will never turn another piece of Lapua brass again. Some still do. I dont know of a single serious point blank shooter who doesn't turn there brass. Lee

Normmatzen
08-25-2011, 06:35 PM
First, thank all of you for your concise, useful information!

Charles E. Ya, I come from a career in Electronic Engineering and Audiology and we accept nothing that is not clinically documented and mostly peer reviewed. Anecdotal evidence is a No-No. But, you are right, never going to get really statistically significant answers to crazy questions like mine. But, your answer is near brilliant! and the other listers added enough to make the total answer basically peer reviewed!
I'm satisfied, when I switch over from 243 WIN to 6BRX, I'm going to use a no turn reamer! Cut for the "blue box" Lapua cases.

Bob Kingsbury
08-25-2011, 11:52 PM
Charles, my point was not well stated. I should have said, that I would prefer a true bushing and die with 1/2 thou variation in brass
than perfect necks and a crooked bushing/ die . Nearly every bushing die I have has required some work and about 1 bushing in
5 are true.

Charles E
08-26-2011, 03:18 PM
. . . Nearly every bushing die I have has required some work and about 1 bushing in
5 are true.Sigh. You're probably right. I'm feeling lazy. So I can just blindly *do* & not have to tax my overtired brain, what do you do to check your dies & bushings?

(To amplify: yesterday I assembled my new Hunter windage top. If I were left-handed, I would have gotten it exactly right. Today I get to do it over. Gimme a recipe & I'll do better.)

(If Bob doesn't see this -- the thread is about over -- I'll repost it as a new topic. Strikes me as rather important.)

Bob Kingsbury
08-27-2011, 12:26 AM
When a bushing die body is made, the process calls for maching at one end then more maching at the other. being made on a production
level, its nearly impossible to have both ends on the same centerline axis. Now the cavity which holds the bushing need not be on
the same axis, because we have clearance all around the bushing. What does come into play is that the surface which the bushing
stops against when the case neck is sized needs to be perpendicular. If any tilting of the bushing occurs, every bushing might as well
be crooked. I generally machine the top face of the bushing end. This is done on a fitted, tapered mandrill. With the top face square,
I make a new top for the die. This will be fixed as to the amount you want to size the neck in length. The surface that mates with
the top of die and the second surface which stops the bushing is now parallel.
Using V-block with very narrow surfaces and indicating at the little radius where the bushing stops on the neck will tell you how
you are progressing. One dead giveaway can be found with a MIC. Measure the bushing for height. If one side is taller, the hole
could only be perpendicular to one end. In the v-block flipping the bushing can give you different runout. Bushing with raised numbers
from stamping need to be dressed with a diamond file.