Donuts vs. no donuts... Pressure Trace data

kombayotch

kombayotch

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This is interesting, so I thought I'd share it.

I've been getting unexplained (formerly unexplained) flyers with my 190 SMK load (308 Win.). This load was awesome when I first developed it. 5 shot groups in the .3's and .4's at 100 yards consistently and many 300 yard 10 shot groups under 1 3/4".

The brass was 7x fired Lapua, annealed after the 6th firing. It was FL sized, setting the shoulder back only 0.001-0.002" each time. I thought my brass all had uniform neck thickness and runout. But, I missed the donuts because the pilot on my case inspector runs in the part of the neck beyond the donut.

Here are the traces I was getting with brass that had been sorted by neck thickness and runout. This was supposed to be the "golden" brass, the best of the lot.

donuts.jpg

"Golden" brass with donuts.

donuts_2.jpg

"Golden" brass with donuts.

donuts_3.jpg

"Golden" brass with donuts.


I was still getting SD numbers in the singles, but the traces were all over the place. Many of the traces had lags in them, so I knew something was wrong. I tried cleaning the bore thoroughly and playing with seating depth. I ended up buying a box of FGMM and it shot fine. So, I knew the problem was in the brass.

I sized some cases, but didn't run them over the 0.336" mandrel. I took the 0.335" mandrel and tried to insert it. Sure enough, it only slid down 3/4 the length of the neck and then it was a tight fit. So, I took a 0.336" reamer, chucked it in the drill and stuck it in the neck. It too only got 3/4 of the way down. A touch of the drill trigger and it went through.

Now, here is the interesting thing: I did this to my reject brass first to see if that was the problem. This is the brass with lots of neck thickness variation and necks that are dinged or scratched on the inside. I use this stuff for foulers and rough sighters... It gave me traces like this:

No_donuts.jpg

"Reject" brass with donuts removed.

No_donuts_3.jpg

"Reject" brass with donuts removed.

No_donuts_2.jpg

"Reject" brass with donuts removed.

Groups shrunk to ragged holes again. The largest group from the "rejects" was smaller than the smallest group from the "golden" brass, over all seating depths.

So, even if you don't want to turn necks, one of those pilots with the donut cutter (K&M???) may be a worthwhile investment. I was just using a regular chucking reamer because i had it. If I were to do it again, I'd just expand the inside of the neck with a mandrel and turn the outside of the necks. It would have been less work overall and I would now have brass with all uniform necks. I think its better to leave the smooth finish on the inside of the necks alone also.

For the newbies, this is what is meant by "donuts":
260AI%20donut.jpg

http://www.6mmbr.com/260AIforming.html

.
 
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For your interest, this is what the FGMM traces look like:

FGMM1.jpg

First shots down a bore that has been polised to the bare metal. First two groups had flyers.

FGMM2.jpg

Next 5 shot group formed one ragged hole...
 
Did you used any particular type of chemical flaw enhancing processes to visual aid the fatigue or is the brass right out of the breech.

Driller
 
So the biggest factor is the difference at the very beginning, in the first couple ms. This is where neck tension will effect the timing of the pressure curve.

At peak pressure, the bullet has travelled about an inch (from its loaded position in the case) so this is where all the "action" is happening and where the variations begin.

It looks like at around 0.1ms is where the bullet is first "pushed" out of the case. The slight reduction in slope of the pressure curve is likely the bullet first starting to move, or "jumping" as pressure builds, overcoming static friction from neck tension.
 
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That picture isn't mine, its from 6mmbr.com. But no, I doubt they used any chemicals to enhance the donut.

Yes, the first part of the pressure curve seems to be the critical one. Anytime I get those delays, accuracy goes to crap. That is probably a reasonable assumption that there is a reduction is pressure initially when the bullet is free of the donut.
 
More great stuff from kombayotch - you need a website so it is visible to a wider audience, and remains visible instead of getting pushed "off the page" on a bb system like cgn.
 
Nice data. I had my first problem with donuts last year in my 6BR using Lapua brass. It's amazing how closely my shooting experience with the brass parallels yours, except the donuts showed up within the first couple of firings. Much as described I had some old Remington 7BR brass, shot many times, that I formed to 6BR for practice and so on and it still doesn't give me flyers or form donuts. I ran the resizing mandrel fully through the neck in Lapua brass and outside neck turned them. Accuracy improved, but the problem did show up again with that particular bunch of brass.
 
I was shooting through a CED with IR. The stats are listed on all but one of the charts above, at the bottom. SD's were 7-9 on the brass with donuts and 3-6 on the brass with them removed.

BTW, the powder was all dispensed on a Chargemaster that has a reducer and tuned software. That makes it accurate to a kernel or two of powder.
 
I had read about donuts and had gone so far as to purchase a K&M reamer pilot for my .308 K&M neck turner tool. After a few tries, I stopped using the reamer, as all I was doing was creating tooling flaws inside the neck. After some reflection (and reading of Zediker (sp?)) I've come to some conclusions.

First - when I have to use an expanding mandrel to use the neck turner, I'll never see an internal donut - I'll see it outside on the neck turning.

Second - I have seen the external donut in my reloading, but I had attributed it to to my neck sizing die not being set low enuf (my bad).

Third - by the time you can see the donut on your brass, you've likely taken it into the more brittle regime - either retire (scrap) the brass or anneal it. Neck turning will help a bit, but your neck tension will have changed as the material flows.

Fourth - most of this 'knowledge' has come out of mandrel expansion of brass for neck turning of a bunch of brass of 'mystery' shootings (I had some older brass of unknown firings mixed in with known firings during a move - oops). The internal sizing button would stick on some cases (the harder, more brittle, ones) which gave me the intiial indication, then the mandrel expansion was tough - even ripped a neck off one case (yes, I lubricate, in moderation).

My learnings - keep cases separted by number of firings, observe the case for the external domunt, don't worry about internal donuts (in unmodified cases).
 
P1040486.jpg


P1040485.jpg


A McDonald's straw alone is useless... What I've done is roll a piece of paper around a 3/16" rod and insert it. I haven't gotten around to machining a permanent one yet. The tube restricts the flow of the powder so that you don't get a big clump of powder putting it over your weight. I've found that different diameter holes work better for different powders.

I adjusted the software parameters to:

HSP_A1 = 8.00
HSP_B1 = 3.00
HSP_C1 = 1.00

Its slower, but it doesn't overflow at all. You can see a single kernel or two put it on the desired weight each time.
 
Other useful tips:

-When you remove the pan to dump it, always check that the readout is -(pan weight).

-Watch for static buildup between the housing and the plastic platform. It causes errors.

-Auto mode is not your friend if you're going for accuracy. It doesn't give the scale enough time to settle on zero.
 
This is why I believe in throating match rifles so that the bullet remains ahead of the neck/shoulder juncture. This especially if dimensions are on the tight side and if necks are turned. Stay ahead of the donut and it's effects are minimized.
Expanding the neck on a mandrel and outside turning is not the best solution because you end up with a thin spot just ahead of the donut. The reamer works best providing it is kept straight. Regards, Bill
 
scout said:
After a few tries, I stopped using the reamer, as all I was doing was creating tooling flaws inside the neck.
Click to expand...

I got mine today and gave it a try. Didn't have any issues with tooling marks. I only ran the expander down to where the donut starts like they suggest (that depth stop on the K&M expander rocks). I ran the cases with my 18V Dewalt on high speed and then ran a worn out bronze brush with 0000 steel wool wrapped around it down the necks right after turning each case. The result was a beautiful, smooth machined finish on both the inside and outside of the necks.
 
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