Runout in size die body - Does it matter? Probably not...

[DownhillFromHere]

Summary:
New RCBS MatchMaster resizing die (caliber 6BR) has verified 0.002" runout between interior top and bottom of the die body, which creates a 0.0015-0.002" misalignment of case body and case neck. This results in a corresponding runout in the seated bullet. In practicality, does this matter downrange? I'm thinking probably not.

This thread is simply for comment/discussion; bullet TIR had been thoroughly beaten elsewhere - but I haven't seen anything about actual runout in the die itself. It may well be that the measured eccentricity is within manufacturer spec - RCBS hasn't responded to my inquiry yet.

Detail:
I recently bought several hundred pieces of new Alpha 6BR brass and a set of RCBS MatchMaster dies (resize, seater). For whatever reason - I've not done this with other calibers I handload - I checked concentricity of case mouth and case body on about a dozen virgin cases. Runout was essentially zero on all of them.

So I was quite surprised to find ~0.002" runout of case mouth from case body in resized brass (AMP-annealed after every firing). So I took the following steps, in order shown, to isolate the runout source.

1. Insured the neck bushing was floating in the size die. It was.
2. Allowed the die to free-float in press threads. Made no difference (shoulder bump consistent +/- .0005").
3. Put the die and .266" neck bushing in a buddy's single-stage press (I'm running a Dillon RL-550). Multiple cases resized, showed exactly the same amount of runout.
4. Swapped neck bushings - used one of his .266" bushings. Made no difference.
5. My buddy - a respected gunsmith in our region - was intrigued. So he put the die body in his lathe to precisely check concentricity at several points on the exterior and interior of the die body (photo). After alignment, concentricity of the exterior body was found to be perfect from top to bottom - but the interior showed 0.002" runout.

I have an email in to RCBS.

Downrange accuracy:
With brass and barrel broken in (once-fired brass, 300+ rounds through barrel), I'm getting consistent 0.3-0.4" 5-10 shot groups @100 with 108gr ELDMs @2810fps, ES 25/SD 6 (measured over 25 rounds). Midrange (500 yard) 5-shot groups 1/3-1/2 MOA. Longer-range (600-1000) at least that good when wind doesn't blow.

The 100-yard accuracy isn't all that great for 6BR, but neither am I using top-tier bullets - very brief testing with Lapua 105gr Scenar-L bullets in virgin brass showed improved grouping compared to ELDMs, and I haven't been able to find Berger 105gr HTs yet.

Bottom line:
Resize die definitely has a bit of runout, but I cannot, so far, definitively demonstrate that the corresponding case body/neck runout does or does not impact accuracy. I'm putting this in the forum as a simply FYI, and am interested in thoughts/observations from the SH reloaders community.

 

[spife7980]

I refuse to measure runout myself because ignorance is bliss but I imagine it won’t matter a bit downrange.

You are invested this far, now you have to prove it one way or the other

Oh, screw it in half a revolution deeper, sacrificing one or two cases live spans, and see if the runout follows the orientation of the die.

 

[DownhillFromHere]

I refuse to measure runout myself because ignorance is bliss but I imagine it won’t matter a bit downrange.

You are invested this far, now you have to prove it one way or the other

Oh, screw it in half a revolution deeper, sacrificing one or two cases live spans, and see if the runout follows the orientation of the die.

Lol, yeah, I don't know why I measured case eccentricity for this caliber when I've never done it before. None of my other close shooting buddies will "go there" either - they won't go looking for trouble. I just posted this because I find it mildly interesting and thought a few others might.

The runout is definitely in the die. The photo is a screen cap from video. Running the dial indicator around the outside of the die at several "levels" bottom to top showed near-perfect concentricity (the one/ten-thousandths indicator barely moved). But running it at different levels bottom to top inside the die showed runout at the top.

The only way I can think of to "prove" whether the little bit of case body/neck runout makes any difference is to borrow a size die that gives me perfect body/neck concentricity and compare rounds sized with it vs. rounds sized with my die. Having tested seated-bullet runout effect in my early days of precision rifle handloading and finding practically none, I wouldn't expect this "flavor" of runout to be any different.

 

[Rookie]

It's 2 thousands. Forget about it.

 

[Hobo Hilton]

Depends on your mindset.

Is everything else within less than .002" ? Weapon, target, etc..

Or are you a person who adds up .002" here, .004" there, .006 here ?

How does the weapon perform..... Target ?

 

[RegionRat]

I don't blame you for being concerned.

If the inspection of that die is valid, there isn't much of an excuse for it and I wouldn't appreciate the die injecting runout into otherwise good brass.

I am just baffled by how it was made and how it got that bad.

 

[MarkyMark007]

why you think that bushing die runout will translate to neck of your brass?

 

[MarkyMark007]

what piece of die has runout? outside to inside?

 

[iceng]

Size your brass, rotate brass 120deg (1/3 of a turn) ans size it again.

Whats that piece of brass measure now on a concentricity gauge ? Bet its good.

Old bencher trick. Size brass, pull ram down half way, rotate brass, send it right back up.

I have a video somewhere of brass going in good, and coming out like a banana, the die was that bad. RCBS..

 

[Haney]

what piece of die has runout? outside to inside?

Read.

 

[MarkyMark007]

--verified 0.002" runout between interior top and bottom of the die body--
--~0.002" runout of case mouth from case body in resized brass--
--neck bushing was floating in the size die--
--exterior body was found to be perfect from top to bottom - but the interior showed 0.002" runout.--

so how exactly he find out that bushing is not straight to body? it is a bushing die!

and again: it doesnt matter if interior of the die is not in alignment to the outside!!! your brass is FLOATING in the bushing, so it doesnt matter if your internal of the die is not straight !!
because brass is floating in the shellholder and will align to the internal of the die, no matter how tilt it is.

 

[Dolomite]

Lol, yeah, I don't know why I measured case eccentricity for this caliber when I've never done it before. None of my other close shooting buddies will "go there" either - they won't go looking for trouble. I just posted this because I find it mildly interesting and thought a few others might.

The runout is definitely in the die. The photo is a screen cap from video. Running the dial indicator around the outside of the die at several "levels" bottom to top showed near-perfect concentricity (the one/ten-thousandths indicator barely moved). But running it at different levels bottom to top inside the die showed runout at the top.

The only way I can think of to "prove" whether the little bit of case body/neck runout makes any difference is to borrow a size die that gives me perfect body/neck concentricity and compare rounds sized with it vs. rounds sized with my die. Having tested seated-bullet runout effect in my early days of precision rifle handloading and finding practically none, I wouldn't expect this "flavor" of runout to be any different.

That would be a great test. Let us know about the results if you do decide to test.

For me I got the best groups with the worse concentricity ammo. About .004 off, go figure, I’m never measuring concentricity again. It only drives me insane.

 

[bax]

Thinking out loud ... you said, "After alignment, concentricity of the exterior body was found to be perfect from top to bottom - but the interior showed 0.002" runout."

Your friend aligned the outside of the die. Since the runout was zero, we can conclude that the die body was round and that your friend did a good alignment job. That is, if the die body is round and you align the outside, then runout will be zero. I had to be. The way that you can tell when you do a good job aligning is if the runout is zero. Watch a guy set up a barrel for chambering.

So the die body was set up in the lathe and you indicated the inside and saw two thou of runout. As far as I know, the sizing part of the die is made about the same way as chambering a barrel - you drill a hole straight through the raw die body then run a reamer into the die body following that hole. So where did that .002 runout come from? Could be that when they made the die, it was not aligned as carefully as what your friend did. Could be that the hole was not straight or not centered. Could be they made cuts that were too long and the heat and pressure distorted the reamer.

I tried to read if you did this - maybe I just missed it - did you try to indicate the inside of the die? Then you could tell if the "chamber" inside the die was round or oval.

A real machinist (not some random jackass like me) can probably come up with other reasons for the problem.

Does it matters ... practically speaking, you stick a piece of lubricated brass into a shell holder and raise the ram to size the case. How much flop / slop is in the shell holder and does that matter? For example, suppose the piece of brass has a slight rim deformation that prevents it from going all the way into the shell holder and the case is offset by, I don't know, maybe 0.010 and the case enters the die a little crooked. Will the force of sizing shove the brass further into the shell holder and fix the alignment problem? Will the case be distorted?

Over the weekend, I resized about 1,000 pieces of Lake City .223 brass - various years. Some pieces of that brass had much thicker rims than other pieces. The thick pieces tightly fit the shell holder, thinner rims had more slop. A piece of brass would slide into the shell holder but also slide side-to-side a little. Does that matter?

In his book, Tony Boyer told a story about a guy selling a concentricity gauge at some match. He didn't know who Tony was and he measured a few of Tony's loads. The tool guy said that Tony's loads could be improved. Since Tony won BR matches as regularly as sunrise he didn't change anything, concentricity was not a problem that he needed to fix. YMMV.

This is an interesting question. As you do more experimenting I will be interested to hear what you find.

 

[iceng]

Its that thing of "you could win by more" .. the gap is closing for good enough vs best..

Aaaand people with production rifles and factory ammo can beat me in PRS matches.