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Are your flash holes centered?

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Unread 03-08-2011, 02:16 PM
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Join Date: Oct 2008
Location: Bryan, Tx
Posts: 284
Are your flash holes centered?

Since it is a rainy day and the range is closed I want to kick around a new subject...at least for me.

I believe whole heartedly that one of the most important steps in first sorting of new brass is to measure neck thickness variation. To do this I use the sinclair case neck sorter with a dial indicator guage. The less variation the better IMHO.

Until recently I have always just visually inspected the flash holes of new brass to make sure they were in the center. However, when neck sorting some new 300WSM brass to neck down, I tried measuring what I call "flash hole runout" by spinning cases on the neck thickness variation guage, but taking a measurement at the case web using both the neck sorter and the sinclair runout guage in tandem. Cases are completely unsupported by the case runout guage, I am essentially just using it as a dial stand. Not a perfect system, but it is stable and it gives real measurements.



The better brass (Lapua, Norma) gave very low measurements (0.001-0.002) pretty consistently while the Win brass gave readings all over the place (0.005-0.015) on just a small number of pieces (a couple even up to 0.020). What this tells me is that either the flash holes are oblong (not likely since the spindle fits each flash hole almost perfectly, or the the flash holes can have tremendous variation as to their location relative to the center of the primer pocket. Measuring this runout tells you if the flash holes are centered in the primer pocket.

So... I have re-invented the wheel but,

Better brass are more likely to have centered primer pockets (not rocket science).
Using a tool to measure this runout is probably more accurate than visual inspection. I am sorting brass by neck variation anyway, so taking a reading at the case web is not a big stretch. Sorting by flash hole runout would likely only be necessary once on new brass, so there really isn't a lot of extra work.

Has anybody else tried to measure this?
Does being off center make a difference in accuracy?
If you have the same tools and some unprimed brass, let me know what you think. Maybe there is a better way to measure this?

One could certainly argue that this might not make much of a difference in long range accuracy. I will be the first to say that reading the wind et al. will make more of a difference, but... nonetheless, here we go. Constructive criticism only please!
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Unread 03-09-2011, 09:56 AM
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Join Date: Apr 2010
Location: Allen, TX
Posts: 2,608
Re: Are your flash holes centered?

I have not tried this. But being somewhat OCD, I guess I'll have to now.

I agree that it may have little effect. But, why not do it if you have time and can afford to set a batch of irregular pieces aside for such things as pressure workup or foulers.

On the other hand, I have often pondered the distribution of powder for a non-compressed load lying horizontal in the chamber. That would certainly vary more than an off center flash hole although a long oval flash hole seems highly undesireable.

I'd be real surprised if the benchrest shooters haven't studied these things to death and I'm pretty sure that loads that approach case capacity are often recommended/preferred for that very reason.

Do you index your brass for the thin side of the case to offset the banana case syndrome?

Perhaps you should index the flash hole to 6 o'clock?

Do you suppose that the offset flash hole is a reflection of overall case concentricity?

If these are the things one worries about, then Lapua may well be the least expensive brass on the market.

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Unread 03-09-2011, 09:09 PM
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Join Date: Oct 2008
Location: Bryan, Tx
Posts: 284
Re: Are your flash holes centered?

Well I repeated some of my tests and get similar data, but not sure exactly what it means...

1) Is the flash hole off center from the axis of the cartridge with concentric primer pocket?

2) Is the primer pocket off center w/ the flash hole in the middle of the primer pocket

3) Are other contributing factors adding to the runout giving a false reading

I get runout on both new and once fired brass. I feel that most likely primer pockets are concentric, and that the error comes from drilling/punching flash holes slightly off center, with some being noticeably off center. If the flash hole is not in the middle of the pocket than it would likely impact the direction/concentration of the flame to some degree. Add that to a horizontal non-compressed load...?

My theory is to sort my brass initially on neck thickness variation, that way I can eliminate the banana cartridge theory. If the brass is the same all the way down, it can't form a banana. My data says that neck thickness variation continues down the length of the case. Spinning cases on a homemade guage with a feeler bar at the case web showed runout there as well. This was not as present in cases with minimal neck thickness variation. If I am shooting cases with neck thickness variation I always index. It is really easy to do. Just give a light neck turn and the side of the neck that isn't trimmed is the thin side! Then mark with a small file. Cases with increased neck variation after neck turning actually increase bullet/neck runout until after the first fireforming.

I have ordered 100 win brass to get (just barely) 50 pieces of usable brass. Good brass (norma/lapua) just cuts down on my work and worry. I can make rem/win/fed brass good, but it takes A LOT more effort. Now add in the flash hole sort, maybe I'd need 150? All of the sudden, Norma brass isn't so expensive.

My thoughts are initially to spin the cases and catch the flash holes that are really off and cull. Where to make the cut will have to be empirical without field trials for now. Simple, easy and effective. Spin once and done and on to the range.
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Unread 03-10-2011, 10:04 PM
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Join Date: Oct 2008
Location: Bryan, Tx
Posts: 284
Re: Are your flash holes centered?

Posted with the BR guys. They suggested fh reaming. Fh reaming (sinclair fh reamer) had little to no effect. In fact it might make runout worse. Maybe it creates an oblong hole? Maybe primer pocket also off center?
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