Advice needed. Pressure issue.

[farmer7]

Hello,

I have a couple of loads for my .308 Tikka 20" that are causing me concern and I dont understand whats going on!

I worked up using the OCW method with 155gr Amax, Lapua brass, CCI BR4s and N140. Found a node at 45.3 and settled on it. Experienced no pressure signs at all. Velocity 2706fps.

I then worked up a load using the same components except this time using 178gr Amax. I worked up to 43.7 but the node was at 42.8 so I settled on that. Average velocity was 2499fps. Temperature was 55ºF.

Now the last few days when shooting using either load I get no pressure signs at all for the first 4 or 5 shots but after that every single case is showing very light ejector marks. Anyone know why? Am I on the limit? Not one single case has the faintest mark until shot 4 or 5, EVERY time! Temperature has been up a wee bit, 64ºF.

I'm shooting about 1 shot per minute and 5 minutes between groups. I thought initally I was heating the cartridges up in a hot chamber but today I chambered just before the shot and still getting ejector marks.

The barrel was cleaned thoroughly and the chamber was dry of any oil or lube.

I know N140 isnt quite as temperature stable as Varget but I cant get that over here currently.

Primers aren't flattened or leaking though. I just dont understand why its not until shot 5 or so. Not coincidence either as its everytime with every case.

Anyone any advice or opinions?

I dont like skating on the edge of safety but could there be something else up I'm not thinking of?

Thank you!

 

[BuzzBoss915]

Can you describe your cleaning regimen? Or, it's possible you just have a very rough bore that fouls easily? BB

 

[Greg Langelius *]

Check the cases with the ejector marks/pressure signs for excessive case length.

 

[Dan Newberry]

I would probably just back off the loads a bit. I've seen this happen (what you're describing), and actually I've had it happen on some of my own loads back in the days when brass was cheap and easy and I was driving everything like I stole it ( )... I believe it has to do with chamber temps rising just enough to speed the powder up *just enough* to put you over the edge. That simply means that the load is too hot.

If you've truly ID'd OCW's for both bullet weights, drop those charges by 3% which should bring you down to the next node. Since you're very likely maxed out at the charges you're shooting, you might be surprised how little velocity you end up losing by this 3% decrease. And even if you do give up a bit of speed, you'll gain a lot of peace of mind here...

It's not "back to the drawing board"... drop the charges by 3% and shoot some groups and see what comes of it.

Dan

 

[Greg Langelius *]

Where load development is concerned, Dan is one of the most knowledgeable handloaders I've encountered. If you haven't studied his process, now would be an excellent opportunity to enlighten oneself.

Greg

 

[Dan Newberry]

Greg... that's very kind of you, but let me say that I remember the days on the old Shooters forum where I picked your brain continually, and learned much from your writings.