What are signs of excess pressure

[Tom15034]

At what I read on here to be a max load, I get hard bolt lift with about 30% of the shots. This was yesterday at ambient temp of 35F.

What are other signs of excess pressure besides hard bolt lift? Someone talked about extractor groove expansion. Someone else mentioned ejector marks. What about case expansion?

Tom

 

[Forum Boss]

What is your cartridge?

What brass are you running?

Is the chamber 'custom' or SAAMI?

How frequently do you resize your brass. The hard bolt lift may just mean it's time to bump the shoulders. But, yes, you do need to check for expansion at various points on the case, extractor groove diametric growth being a sure sign of high pressure. Also look for a bright spot where the ejector was and look for deep cratering on your primer pocket.

IMPORTANT SAFETY RULE: Max loads vary from gun to gun. A "practical max" load in one gun may be unsafe in another. To give a real world example, Jason and I both shoot the same bullets from the same lots in 1:8" twist 6BRs. He can run about .7 grains more powder than I to achieve the same velocity.

 

[Tom15034]

I have a 6mm BR 268 neck, Rem 700, 28" 8 twist barrel. Cases were once fired Lapua FL resized. Load in question was 30g Varget, 107g Sierra MK seated 10-20 off lands, CCI 450 primer. Loaded rounds all had neck OD of 265. Bolt lift was fine at 29.5g.

I have the bright spot at the ejector hole and cratered primers.

What is extractor ring expansion?

What is effect of cold temp? Would I get higher pressure at room temp?

Thanks

Tom

 

[rstreich]

I have the bright spot at the ejector hole and cratered primers.

What is extractor ring expansion?

What is effect of cold temp? Would I get higher pressure at room temp?

Yeah, you're too hot, Tom. The bright spot at you ejector hole is a sign that the brass was extruded into the ejector hole and then scraped by the edge of the ejector hole as you open the bolt. Cratered primers can be a sign of too much pressure or a couple of other things like a poorly fitting firing pin or a too weak firing pin spring. I'm not sure what Paul is referring to with extractor groove growth.

Athe temperature goes up, the pressure is going to increase. Even your load of 29.5 might be too hot for 70 degrees.

 

[Tom15034]

Thanks, I noticed one of the cases that had 29.5g powder shows the bright spot from the ejector hole. Should I scrap these cases?

Tom

 

[Forum Boss]

With over-pressure, the diameter of the web and the diameter of the extractor groove will grow. A little bit of expansion is no big deal. If your cases have grown so much that the case is hard to put into a shell-holder, then yes the brass is probably not salvageable.

But, if you're just seeing a hint of ejector wipe I wouldn't worry too much. Normally, 30.0 grains Varget is NOT a gear buster load. But we do see lot-to-lot variations and you may have a short throat.

If you are seating a full .020" off the lands, you may find that you can actually REDUCE pressure signs by going about .010" into the lands--try that with a couple cases. This is because, by moving the bullet forwards, you are increasing effective case capacity.

I would also look at your throat. Could be you have a carbon ring build-up that is causing pressure to increase. Jason encountered this problem at about 500 rounds.

Also, what brand of resizing die are you using? Some size the case more than others and that can make a difference.

What is your Cartridge OAL length,from case base to bullet tip). That will give me an idea of how your rifle is throated. If you have a very short throat then 30.0 could, indeed, be a pretty hot load.

The Temperature, according to Hodgdon data, should not be causing the pressure problem: http://www.hodgdon.com/smokeless/extreme/page3.php#308win

Varget is NOT one of those powders which actually shows MORE pressure at low ambient temps.

 

[Tom15034]

Thanks for all the help!

Rim and groove diameters of cases that had 30g are around 1 thou bigger than those that had 29g or new brass. They go into a shell holder fine.

Not sure about carbon in the throat. I'll hit it with some Iosso.

Sizing die is a Forster FL. AOL with the 107g Match King is 2.343 touching the lands. The previous owner said the throat was made for 95g Berger VLD's.

Tom

 

[Forum Boss]

The 95 VLD has a super-short bearing surface, so it does look like you have a fairly short throat. I can barely shoot those in my gun which has about .085 freebore measured the way Kiff does.

Take a moderately short throat, maybe a faster than average lot of Varget, and seating .020 away from lands, yes I can see how you may have some pressure.

If you hit the throat... do it gently. What works well is a patch over an old, fairly worn out brush that's slight small in diameter.

What I'd do personally is go put the bullets .012 or so into the land, and start with a 28.5 gr load. You don't have to load a lot of rounds... just check for pressure. Once you get to 29.7 or so, work in up in .2 or .3 grain increments. I bet you'll be able to shoot that 30.0 grain load if you seat longer. But plenty of guys have found their best accuracy at 29.5 or thereabouts.

 

[Tom15034]

OK Mr. Moderator. Thanks again for the help.

Tom