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I need help with pressure problems

 
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  #1  
Old 05-04-2012, 01:19 AM
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Posts: 241
I need help with pressure problems

Hello all...

I am shooting a 6.5x47 and am having some pressure problems.

I am using Lapua brass
H4350(and a test string of 2000 MR)
Federal match small rifle primers
140g A-Max's

I have loaded all the way down to 36 grains of bot 4350 and 2000 MR shot it over the CE M2 and registered velosity all the way down to 2477(most people are between 2840-2940 with this bullet) and even with the lightest charge i am still getting extractor swipe and slighty heavy bolt lift. Every round that has been fired(yes im hard headed and keep trying) has the swipe....I just dont understand..

The gun was built by a great builder with a great reputation and taking it back to him will be the next step.

I have measured the new brass(i did not trim it) but it measured 1.8435 so unless my chamber it short they should be great.

any ideas? someone suggested using varget but with velosity down around 2400 there should be no pressure.

PS the gun is a shooter...I have shot three three shot groups that were all under .2.
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Old 05-04-2012, 02:19 AM
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Join Date: May 2009
Location: Australia
Posts: 733
Re: I need help with pressure problems

I have no experience with the 6.5 x 47 lapua. However there is a few things you should check.
Lubricate the bolt lugs and cocking cam with molly grease.
Check the powder in the tin is the correct type. Maybe buy another can of 4350 to make sure .
Scrub the barrel and chamber out extra well with a good powerful copper solvent . Inspect the brass for rings that might indicate a rough chamber .
If so polish out the chamber .
Polish up the brass extra clean.
Size the brass for a neat but easy fit so it does not crush into the chamber because that can cause hard extraction.
Make sure the extractor is not bedding into the case rim and stopping the bolt from turning easy initially .
Some guns just will not digest as much powder as others.
Make sure your Chronogarph is reading accurately , test some other gun and known ammo.
Using Varget should make things worse as it is faster than H4350.
Don't just go by bolt lift also check primer flattening and cratering to see if it all matches .
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  #3  
Old 05-04-2012, 02:26 AM
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Re: I need help with pressure problems

I would say that you have a short range benchrest type chamber designed for light weigth flat based bullets or a throat that requires neck turned cases.

The chamber most likely has almost no lead. ie a very short throat. I have had the same problem with a 6mmBR that had a short throat cut.

Try a bit lighter bullets of the Berger VLD type with a short bearing surface and secant ogive. Be careful to seat the bullets to be just short of the lands or to just kiss the lands.

If after trying the lighter bullets, it still has pressure problems, take it back to the gunsmith.
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Old 05-04-2012, 07:28 AM
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Re: I need help with pressure problems

It's very odd for premature pressure problems from that cartridge. 1st I've ever heard of it.
I would measure near the webs, headspacing, and necks, before and after firing to determine the problem.
Just compare your new prepped to once fired(and not yet sized).
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  #5  
Old 05-04-2012, 01:42 PM
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Re: I need help with pressure problems

Thanks for the ideas.

I spoke with my builder this morning and he suggested I go to a 130g Berger. Although I could see the Berger having a little less pressure because of the bearing surface I don't see it solving the problem. I'm 400 fps slow not 50. I ran my brass through a mandrel die to ensure the necks were all the same, but did not turn anything. I didn't think much about it but I have a little trouble closing the bolt so it might be a tight chamber( new brass). So how do I check that sort of thing?
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  #6  
Old 05-04-2012, 05:17 PM
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Re: I need help with pressure problems

Measure near the webs, headspacing, and necks, on before and after firing to determine the problem.
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  #7  
Old 05-04-2012, 10:01 PM
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Join Date: Oct 2008
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Re: I need help with pressure problems

Measure the neck dimension of a loaded cartridge and compare it to the neck dimension of a fired case. You should have at least .003 difference. If the case neck can not expand to release the bullet, it can cause a pressure spike. You can also use a permanent marker on a loaded cartridge to see where it is making contact when chambering.

Did the gun maker stamp a neck dimension on the barrel?
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