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338 Lapua - case expansion and sticky bolt ?

 
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  #1  
Old 08-02-2010, 12:35 AM
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338 Lapua - case expansion and sticky bolt ?

I had a custom 338 Lapua built in 2009. I am having ongoing problems with sticky bolt and difficult extraction, even with mellow loads of 87.0 grains of RL 25 and 87.8 H1000 under 300 SMKs (seated -0.040" from lands) that go around 2700 fps out of my 28.5" barrel. From my research, this is by no means a hot load, but all indications are that I am getting excessive pressure. I seem to get similar results at even lower charges. Apparently, the cases (lapua head stamp) are expanding larger than the chamber after firing.

I have been resizing the brass using a redding body die to set the shoulder back around -.001 to -.002" measured with Sinclair bump guage) while ensuring the case head is sized as well. After I FL size the case head, it "pops" when I raise the press handle past the case head. After FL resizing, the brass chambers without issue. However, after one firing of the FL sized brass, the case/bolt sticks, often requiring some extra force to lift the bolt past the top of the extraction cam and sometimes even when extracting the bolt. Feeding once fired brass will typically lock the bolt before fully chambering the round, and it is very difficult to extract, often requiring hammering the bolt the open it. Only neck sizing is not an option at all. I trim all brass a 2.714" every couple of shots (seems like too often). The rifle is very accurate with these loads, but I'm scared to try and get any more velocity out of it. As it is, the sticky bolt extraction and swelled cases that won't chamber after one firing are causing me grief/concern.

Any thoughts or suggestions? thanks
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Old 08-02-2010, 07:28 AM
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Re: 338 Lapua - case expansion and sticky bolt ?

I would verify that you have adaquate room for neck expansion. If the reamer cut the area the outside of the case neck chambers too tight it can cause pressures to run high. I think you could neck turn a couple cases and fire them with the same powder charges you have been trying to see if the pressures drop. Also make sure your brass is trimmed to the right length and check your scale.
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Old 08-03-2010, 06:48 AM
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Re: 338 Lapua - case expansion and sticky bolt ?

I'm assuming your measuring with a caliper, are you confident in their accuracy? IMHO I test the calipers on an item of a known dimension and see if they are accurate.
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Old 08-03-2010, 07:08 AM
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Re: 338 Lapua - case expansion and sticky bolt ?

Quote:
Originally Posted by 3006savage View Post
I would verify that you have adaquate room for neck expansion. If the reamer cut the area the outside of the case neck chambers too tight it can cause pressures to run high. I think you could neck turn a couple cases and fire them with the same powder charges you have been trying to see if the pressures drop. Also make sure your brass is trimmed to the right length and check your scale.
+1
This has been a problem with some custom with tight necks.

Also make sure that you are not seating the bullets on or close to the lands.

Good luck.

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Old 08-03-2010, 07:14 AM
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Re: 338 Lapua - case expansion and sticky bolt ?

I have seen this happen with brass being used in two different rifles,did you start with new brass also i have had a batch of primers that spiked pressures,just a few things to help eliminate.
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Old 08-03-2010, 08:56 PM
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Re: 338 Lapua - case expansion and sticky bolt ?

I am confident in the accuracy of my neasurements - I geared up with all the right tools to try and figure this problem out. Plus, the brass is not showing the ring that would suggest a headspace problem.

The reamer specs show the neck diameter at the throat to be .373" - whether that is what it actually is, I can't say ? What would be a typical (SAAMI) case neck chamber diameter?
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Old 08-03-2010, 09:09 PM
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Re: 338 Lapua - case expansion and sticky bolt ?

Measure a loaded round neck and a fired case neck. Also see if a bullet fits in a fired case.
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