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Be careful, myths about pressure signs.

 
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  #1  
Old 01-07-2008, 01:13 PM
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Be careful, myths about pressure signs.

I did a little experiment over the weekend. I ran a little ladder test with my Savage 300WSM, 200gr Accubond and H4350. I started getting a sticky bolt at 61.5gr and 2820fps. I went on up to 62gr and called it good. I was also getting a pretty good cratering around the firing pin that I felt was due to the poor firing pin fit and the firing pin hole being a little funnelled due to Savage tumbling the bolt heads to remove sharp edges. I wish I had a picture of the bolt face, but here is a picture of 3 brass that was shot at 61.5gr before I made any changes. You can see a well cratered primer and also some shine from the boltface on the picture below.



So I decided to make a few changes and see what kind of changes I could get in the primer/extraction (typical pressure signs that everyone talks about).

I took the bolt head over to a friends lathe and faced off .003", which removed the slight dish in the boltface and also squared off the firing pin hole.I re-headspaced the barrel and reran the tests. The cratering looked much better (hole is still oversized, but visually it looks better). The hard extraction was still occurring, so I took the rifle apart and lightly lapped the lugs. I then reinstalled the barrel, headspaced it and put everything back together.

Back to the range I went. In the original test, I started getting a sticky bolt lift at 61.5gr of powder. In this final test, I went all the way up to 63gr with absolutely no bolt lift pressure and a much better looking primer, as can be seen in this picture at 63gr's. No marks were picked up from the boltface at this loading with the 'cleaned up' boltface. The few marks that can be seen are from previous loadings of this same brass.



So, be careful when looking at these 'pressure' signs, as they are relative to everything that is happening on your rifle and shouldn't be taken as absolutes. In neither of the pictures above do you see a flattened primer that no longer has the rounded edges, but that sign can also be misleading, as different primers and even different lots of the same primer can be made from metal that is harder or softer.

AJ
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Last edited by AJ Peacock; 01-07-2008 at 01:18 PM.
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Old 01-07-2008, 01:20 PM
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This has been a topic on other boards I frequent, and much of the same conclusions have been drawn.

The "typical" pressure signs are NOT a good indication, and at best, are only a very gross estimation of high pressure. More like a CAUTION sign than anything.

At least one load I shoot, the 6.5 Grendel, has proven to me that watching for pressure signs has almost NOTHING to do with what is actually happening in the rifle. Oftentimes Grendel brass shows NO pressure signs, and yet people have actually broken bolts with those rifles when they have been pushed.

Be very careful!

Bill
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Old 01-07-2008, 02:40 PM
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Sounds like the boltface was not square with the lugs. With this, partial excess HS and difficult bolt lift while wiping the now out of square caseheads.

With early load development, I normally bring calipers along and watch for .0005" growth near the case heads(compared to any previous shot) as I go upward. Once I see this growth I know I've reached MY max.
I back off From there, because the brass wouldn't last, and normally this is too much pressure anyway. Normally I get the primer flattening before this.

Good Post. Great info
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Old 01-07-2008, 03:08 PM
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Quote:
Originally Posted by Mikecr View Post
...

With early load development, I normally bring calipers along and watch for .0005" growth near the case heads(compared to any previous shot) as I go upward. Once I see this growth I know I've reached MY max.
I back off From there, because the brass wouldn't last, and normally this is too much pressure anyway. Normally I get the primer flattening before this.

Good Post. Great info

Thanks, here is a document that you might like to read. In this document, the author does a pretty good job of showing that pressure ring expansion (the thing you are measuring) and also Case Head expansion do not correlate close enough with significant changes in pressure for use as pressure indicators.

http://www.shootingsoftware.com/ftp/...%2019%2004.pdf

AJ
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Old 01-07-2008, 06:10 PM
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Helluva brochure..

This is what I know:
Whether the pressure is actually 80kpsi or 40kpsi, as long as I don't have a problem, it isn't. I do not want my brass growing, and I don't care to beat my bolt open. By backing off from these conditions, I have never run into an issue.

Your bolt face created problems before it should have, and you had signs of trouble. You were wise to investigate it. So everything there is fine, and it really matters not what the actual pressure was.
You were well under max.
Right?
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Old 01-07-2008, 06:17 PM
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Does anyone make an oversized firing pin for the Savages?
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Old 01-07-2008, 07:14 PM
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AJ,

Was the sticky bolt a lift problem or extraction (pulling back) problem? My STW has a problem of being able to pull the bolt back. The lift is fine. From what you described, if my bolt face was not square this might put pressure on the floating head and cause it to jamb. Does this sound correct to you?
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