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Pressure Signs
Posted: Mon Jan 17, 2011 8:58 am
by 20series
All
I'm after some advice, when I started my load development for the new .308 I was shooting brand new Lapua brass, with no massive issues with pressure. 45gns Vit 550 under a 200gn SMK set 10 thou off the lands looked very promising.
Having shot all the new brass I neck sized and trimmed back the cases to 2.008, I then reloaded the cases with the "good loads" to chrono them, however they are now showing flatter primers with marks on the back of cases.
Now I am assuming I've just been a pillock and should have fire formed the cases first and then proceded with development on the second firing
Any (helpfull) suggestions please.
Alan
Re: Pressure Signs
Posted: Mon Jan 17, 2011 9:59 am
by ovenpaa
I Fireform with the same load as I shoot with in my 7mm and accuracy does not seem to be very different. I have not seen over pressure signs with the 7mm.
With .308 I also use Lapua brass and expect to see some primer flattening and light strike marks on the case head. Over pressure indicators for me are moderate cratering followed by hard bolt lift, I back off half a grain if the bolt is hard to lift.
Re: Pressure Signs
Posted: Mon Jan 17, 2011 12:59 pm
by 20series
I think I'll load a few batches at lower loads and repeat the grouping exercise, the groups on Sunday were nothing like as good as before either.
Just a thought, I set all the OALs for a 10 thou jump to the lands, how much difference would making the jump smaller or bigger make to the pressure??
Alan
Re: Pressure Signs
Posted: Mon Jan 17, 2011 1:05 pm
by ovenpaa
Increasing the jump will increase pressure, reducing the jump will accordingly reduce pressure until you are jammed where it will spike again. Ten-fifteen thou can make a significant difference. Can you post a picture of the cartridge head and primer and was the bolt lift OK?
Re: Pressure Signs
Posted: Mon Jan 17, 2011 1:26 pm
by 20series
The bolt lift was OK
I'll put some pics up later, I'm at the small-bore club tonight so it'll be tonight.
Alan
Re: Pressure Signs
Posted: Mon Jan 17, 2011 5:06 pm
by TobyH
I'm interested to see the results. I was talking to a gunsmith about over pressure on sat and would like to see what it looks like, especially as I'm going to be making some warmer loads in the near future...
Re: Pressure Signs
Posted: Mon Jan 17, 2011 5:48 pm
by 20series
Re: Pressure Signs
Posted: Mon Jan 17, 2011 5:59 pm
by ovenpaa
Heh! People do go hotter but I would be inclined to drop it slightly especially given the ambient temperature right now.
Re: Pressure Signs
Posted: Mon Jan 17, 2011 7:02 pm
by TobyH
What exactly are you looking for when judging that?
Re: Pressure Signs
Posted: Mon Jan 17, 2011 7:35 pm
by ovenpaa
The primer face has a smooth surface and has flowed out into the primer pocket at the edges, the indentation from the firing pin has slightly raised edges (Cratering) and I can see slightly brighter witness mark where the case has moved back and hit the bolt.
I have certainly seen worse, in fact I have a few sitting next to me right now, however they are cases from test firings at the proof house so are going to be 5-7.5% above limit.
