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Amazing What Effect .001" Has On Chamber Pressure


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#1 Desert Fox

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Posted 06 May 2012 - 09:27 PM

Ever since I fired my first shot from my 6.5-47 Lapua Target rifle, I'd began to noticed an erratic pressure spike that came and went. I can be shooting two of the reloads, with exact same charge of powder, and one brass will show extractor marking whereas another would show none. I also noticed large variation on my SD and ES. That leaves me scratching my head. So began another investigative search for the likely culprit. I began by measuring the neck diameter of the once fired brass. I found that the average reading was around .290". The fired brass brass is tight enough that it will hold the bullet snugly. A typical brass will have a spring back of around around .001" so that tells me that my chamber neck size is probably .291" (this was later confirmed by my smith during my conversation with him). The next thing I did was to measure the brass neck wall thickness. The measurement I'm getting from my Sinclair Neck wall thickness gage varies between .0125" to .0135". I also measured several 130 grain Berger VLD's and they're consistently measured .2645". The average neck diameter measurement of the loaded round is .292". That made my loaded round .001" larger than my rifle chamber neck diameter. Removing high spot on the brass neck wall got me to my desired wall thickness of .0125". My reloaded round now measured an average .2895 to .290". My last trip to the range did confirm that problem had been solved. Posted Image Posted Image Posted Image Posted Image
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#2 Jason

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Posted 07 May 2012 - 06:50 AM

Interesting! Thanks for sharing that!

#3 ratassassin

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Posted 07 May 2012 - 07:10 AM

Yes, thanks for sharing.

#4 KNOCKED UP

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Posted 07 May 2012 - 07:19 AM

What they said.Thank you.Tom

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#5 Frank

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Posted 07 May 2012 - 09:43 AM

The fired brass brass is tight enough that it will hold the bullet snugly.

Yeah, that exclusively would make one think there is likely something wrong.Great post and Pics... Thanks for Sharing

#6 microtus

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Posted 07 May 2012 - 11:16 AM

NIce write up for the issue and correction. For the reloading dummies like myself, can you put up a picture of the extractor markings that lead you to identify the problem to start? From what I've seen online, some extractor marks are very evident while others I'd probably never catch. Most of what I've reloaded so far is in .45 colt, and recently .243 which I haven't had a chance to bust a cap yet. With luck a .458 socum in another month or so.
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#7 Desert Fox

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Posted 08 May 2012 - 04:25 PM

Here's one example. Not sure if you can see it.Posted Image
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#8 Bisley

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Posted 08 May 2012 - 05:09 PM

That would be the ejector (pin), not the extractor (claw) wouldn't it? Not trying to nit-pick you, just clarifying in case others are looking for extractor marks on the side of the case and the exractor groove :fireworks3: .Posted Image

#9 tawnoper

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Posted 08 May 2012 - 06:48 PM

talking about the scrape mark over the 6.5
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#10 Desert Fox

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Posted 08 May 2012 - 07:41 PM

Bis, I stand corrected. You're right! I mean the ejector mark.Tawnoper, The scraping was from the sharp edge of the ejector hole. The corner edge of the ejector hole get really sharp when Jim squares a bolt face. Due to excessive pressure, portion of the case head flow slightly into the ejector hole. The scraping on the case head happens when turning the bolt to withdrew the fired case. This was what baffled me during my load development because it only happened intermittently.
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#11 lif2fsh

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Posted 08 May 2012 - 07:41 PM

Really nice write-up and some nice shootin, but that is one funny looking cartridge.

#12 Bisley

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Posted 08 May 2012 - 10:40 PM

This was what baffled me during my load development because it only happened intermittently.

I bet. That definitely ain't supposed to be there! Good eyes too, I don't think I would have even noticed it with my eyes :signlol2iu: . And probably would have taken a decade to figure out actual problem. You must be loading pretty hot, or right on the edge. But seeing how it shoots, I wouldn't change a thing! Nice.

#13 Desert Fox

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Posted 09 May 2012 - 03:55 AM

This happens even on a mild load. After removing .001" off the neck wall, the problem disappeared.
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#14 tawnoper

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Posted 09 May 2012 - 05:00 AM

Tawnoper, The scraping was from the sharp edge of the ejector hole. The corner edge of the ejector hole get really sharp when Jim squares a bolt face.

That is a classic sign of pressure getting up there - stiff bolt lift. It leaves those scrape marks.Usually a tight neck isn't a problem on factory produced rifles...but can be on a custom, where you can specify one if you like. If the neck are not turned to match they really clamp down on a chambered bullet and won't let go till the pressure really gets up there. If the difference is much more than you experienced bad things happen.
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#15 Desert Fox

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Posted 09 May 2012 - 10:12 PM

But seeing how it shoots, I wouldn't change a thing! Nice.

One thing about Gruning rifles. It will shoot no matter what you feed them. This last range trip I shot 4 different load starting from 40.9, 41.2, 41.5 and 41.7 and they all clustered together.4 shots @110 yardsPosted Image3 shotsPosted ImagePosted Image5 shots @ 110 yardsPosted ImagePosted ImagePosted ImagePosted Image
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#16 Bisley

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Posted 10 May 2012 - 03:42 PM

:D My first reaction after seeing your beautiful rifle was "Man that's got to be a real bear to carry around" :( . Talk about a one-track mind. I always make it clear when I talk about reloading, guns, etc., that my family and I hunt solely, never match, or long range, and hope it is never taken as any disrespect at all to the match and long range shooting guys. It is meant purely for the fact that in what we do, that much precision and $ on both rifle and equipment in not as necessary. Hunting really is a whole different ball game for sure. And that being said, I must also tip my hat and admire the precision and work the long range and match guys put into their sport and equipment. Very impressive to say the least. And also a great source of info (such as this one) for the rest of us obviously. And while it may be worlds apart in many way, it is also very similar in others. Especially in the fact that neither one can be done from within a city. Both really need to be done in the outdoors. The reason I mention this is after reading so much about hunting usually being the first (and many times only) thing talked about in the gun control fight, seeing setups like like Desert Fox's are a great example of one of the many other reasons for keeping them. Nice shooting, and beautiful rifles. You seem to do consistently what a guy like me would hang on his wall to brag about. Good job. I know a ton of effort goes into the science of your shooting, and it shows. :yahoo:

#17 Desert Fox

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Posted 11 May 2012 - 10:01 PM

I know a ton of effort goes into the science of your shooting, and it shows.

Squeezing every ounce of Accuracy in a rifle take a lot of tedious work. The following illustration are some of the process.First is the use of a neck sizer die like the one on the picture to size the neck just enough just to hold the bullet.Posted ImagePosted ImagePosted ImagePosted ImagePosted ImagePosted ImagePosted ImagePosted ImageThen make sure the loaded cartridge will have no more than .002" out of roundPosted ImagePosted Image
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#18 microtus

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Posted 12 May 2012 - 05:40 AM

Thanks for the pic of the ejector mark (and the entire thread for that matter). I would have had to put on my reading glasses to catch that mark and without seeing it show up on more than one case would probably never even notice it.
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#19 tawnoper

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Posted 12 May 2012 - 06:05 AM

Good write up DF. You have some nice stuff.
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#20 Desert Fox

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Posted 12 May 2012 - 07:43 AM

You're welcome bro.I know it looks like a lot of work! but if your trying to sling 25 bullet into a 10 inch circle at 1000 yards, then every conceivable variables should be eliminated or at least minimize. I do these so that when it's my turn to shoot, all I have to worry is wind and my nerve. :)
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#21 Fjold

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Posted 12 May 2012 - 08:29 AM

I saw the same thing with my last 22.250 build. I had it chambered with a tight .250 neck which is supposed to be large enough not to have to turn the necks. My first shot with a mid powered load and a 50 grain bullet was over 4,300 fps and the bolt lift was sticky. I mic'ed my loaded rounds and found them measuring .248" - .252". Now I turn all the case necks to .248" which is just a light cleaning cut but it sure adds a lot of case prep time when I get new brass.
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#22 Bisley

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Posted 12 May 2012 - 10:43 PM

I do these so that when it's my turn to shoot, all I have to worry is wind and my nerve. :good:

Yeah, because that's the easy part :smiley-outta-here:I couldn't imagine if reloading shotgun took that much effort!!! I'd probably never hunt again :rofl2: . That must be really nice living right down the road from WEGC though. I know it sure has to help for load development <_<

#23 Desert Fox

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Posted 15 May 2012 - 07:42 PM

Went to the range Sunday morning to finalized the load for 6.5-47 Lapua on the upcoming Memorial Day 1000 yard shoot. I'm hoping to better my 3rd place finished last year. I was able to reach my velocity goal of 2900 fps for the 130 grain VLD.

5 shot group @110 yards

Posted Image

3 shot prone using bipod to check if there is any zero shift. There's no change in POI. I attribute that to the stability of the AI stock. The rifle is ready! Not sure about the shooter however.

Posted Image
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#24 homemade

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Posted 17 May 2012 - 07:46 PM

some very good info

#25 rude robert

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Posted 01 August 2012 - 05:14 AM

Nice groups, Is the butt of the rifle in your shoulder when you shoot?
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#26 Desert Fox

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Posted 01 August 2012 - 06:01 PM

Nice groups, Is the butt of the rifle in your shoulder when you shoot?


Your mean free recoil shooting? If so, then the answer is no. I don't do Benchrest.
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#27 ShooterJohn

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Posted 01 August 2012 - 06:32 PM

Great post DF! You need wheels on that gun of yours. ;)

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#28 Desert Fox

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Posted 01 August 2012 - 07:33 PM

Thanks John.

I do like heavy guns. They're very forgiving and very stable to shoot. I've hunted with my 300 Win Mag Model 70 Laredo for a long time and it weigh a tad over twelve lbs.
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#29 rude robert

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Posted 02 August 2012 - 04:24 AM

Very nice
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