Help please! Pressure

[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!

 

[jonbearman]

What does your book say as far as max loads? Temp can affect pressure alot but 10 degrees is doubtful unless the ammo was allowed to sit in the sun all day and heated up alot.Go to the VV site and see what it says.You may be right on the ragged edge of the loads just being too warm.I just checked and you are over the max by about a grain on both. Back off a grain.

 

[Webster]

The Berger manual max load for their 155 gr. bullets is 43.7 grains with N140. The Sierra manual list 44.7 grains with N140 as max with their 155 Match Palma bullet. E-mail Berger or Sierra and ask them the questions you have about how far you should be below the signs of pressure. Someone else just posted a reply the the signs of pressure we look for like ejector marks, primer crater, bolt lift and head swell are signs that you are well past the pressure you should be be at. Not just a little above the safe limit. If you lower the powder charge a little and these signs of pressure go away it doesn't mean you are at a pressure level that the gun was designed to be at. You are just below the level that the cases are not damaged. You could probably shoot thousand's of rounds above the the factory determined max pressure without problems. There is a pressure safety margin that puts the rated pressure below what you could get away with. I read once what the factory proofing round pressure was it's a big number.

I would like to see more info and discussion on this by ballistic engineers and not opinions of what shooters believe is OK. Shooters only know what they get away with without getting into trouble.

 

[bigedp51]

The H.P. White Testing Laboratories has material published about their testing methods and procedures. Below is from H.P.White Laboratories.

And a engine tachometer has a red line for a reason, once you go over the rated rpm you are over stressing the engine and it can come apart.

 

[CatShooter]

Hello,

"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!

Your rifle is talking to you, and you are not listening!!!

 

[farmer7]

Thank you all for your replies, all very useful. The reason I kept on going to where I finished up was I didn't have any pressure signs at all. Also VV load data I keep hearing is notoriously conservative. BUT I've been stupid obviously and this sums it up perfectly:

[/quote]
Your rifle is talking to you, and you are not listening!!!
[/quote]
Thank you for that simple post it hit me hard!! ;D

I'm going to start over and find another node lower down. Its a shame though as it was shooting blinking well!!! But I love this rifle and dont want to abuse it either.

Thanks again.

 

[Webster]

Great comment. I think most people think barrel or action failure is from a single large overload charge. Crack propagation/fatigue failure will do it. I read an article that said the cartridge case is the weak link. Case head separation and gas blowing backwards. I have heard of bolt extractors getting launched at high velocity. Fire cracking is an ideal site for cracks to start growing. Fortunately blowing guns up is a rare thing.

About a week ago a dumpster caught on fire at my rifle range. A lot of people toss loaded ammo and primers they don't want into the waste cans which get put in the dumper. It was a fireworks show. Lucky the dumpsters are steel walled.

 

[338 Mollett]

If you do not have a bore scope, find someone that does. Rifles are like babies ,when they cry we know something is wrong but they can't tell us. Make sure there is no carbon build up.And yes after a few firings brass may show sensitivity toward border line loads. Start with a clean barrel and work backwards ,Good Luck!