Help me read pressure signs!

J

jonnefudge

New member
Hi!
I´m new to the forum and reloading. I've been trying out this load and I hope you guys can help me look if there is any pressure signs on that I should worry about. See following pics. I haven't had any problems with a sticky bolt. As far as I can see the only sign of any concern is the black dot onone of the primer pockets, indicating a tiny gas leakage. The brass i'm using is Lapua och the ones in the pictures have been fired twice.

gasleakage.JPG

STRECK.jpg

platta2.jpg

/Jonnefudge, Sweden (King of the north :D)
 
Welcome to the forum.

I can't tell without handling it in person, but the black dot looks like a brass oxide stain. Not uncommon. Did you inspect and make certain it wasn't there before you fired it? Even if it is a gas leak, when I was doing a lot of service rifle match shooting and working on club guns that were heavily fired with Lake City ammunition, examining the boltfaces always revealed some pitting in a primer-diameter circle around the firing pin because even new ammunition loaded to safe pressures occasionally sprouts a small leak around the primer cup that can do a little gas cutting (the pits). Shoot enough and you get some.

The primers look fine. No mushrooming or cratering. So I don't think you are over book pressure at all.
 
The absolute best way to measure pressure is to get a tool that can read it. Since that is quite expensive and beyond the scope of most handloaders, a chronograph can be a stand-in and affordable to most anyone that had the money to purchase a press, dies, and reloading components. Just chrono the rounds and compare the velocity to what you see in published loads. Using a program like quickload, along with comparing your velocity to published load data, is a pretty reliable method of getting a good pressure estimate.

With that being said, fired brass can and will show clues of over-pressure. It's a bit like reading tea leaves though. For example, a common indicator is "flattened" primers or "cratered" primers. Well, even factory ammo will show flattened and cratered primers with higher pressure loads like your belted magnums and even some higher pressure non-magnum cartridges. Federal brass is notorious for being soft, and I have found that about 3 or 4 reloads is all I will get before the primer pockets become enlarged and primers are loose. Loose primer pockets are supposed to be the last pressure sign before case failure, and it's recommended to back off at least 10% if not more if you experience this. Yet, my reloads that are well within published standards will do this to federal brass after several firings. So... in short I see nothing of concern, but don't take that to mean that you are safe.
 
Agree with Unclenick, it looks more like a stain than a leak. If it were me, and I had no issues with sticky bolt or hard extraction I would be satisfied that the rounds were safe.
 
I should have said, not over pressure for the gun and primer. The primer doesn't tell you absolute pressures because it's not calibrated in any way.

If you get into reading pressure, the Pressure Trace works pretty well, but it does cost as much as a gun. But that's getting pretty far into the weeds for a new handloader.
 
I thought Lapua och" meant "Lapua, ouch! it is expensive!", but googling, I see that is part of the company name.
My father was born into a Swedish only speaking family here in Seattle in 1921. He got 36 gun related patents.
My cousin immigrated from Sweden, worked for Norma, can do internal ballistics calculations. He is got another patent this year in normalizing for force inside digital calipers.

I am the family idiot, an engineer without patents and can't speak Swedish.

But I did learn some engineering along the way.
Good engineers help dumb engineers by asking the same question over and over and it breaks them down, "What is it you are trying to accomplish?"

If I am not selling 308 ammo to the general public, but handloading for myself, I do not care WHAT the pressure is, that would be a red herring for me, the dumb engineer, to get stuck on.

As I work up a 308 load, I should be looking for the threshold of long brass life.

Here is a 308 work up I posted here 6 years ago
http://thefiringline.com/forums/showthread.php?t=429589
 
"...indicating a tiny gas leakage..." Nope. No such thing as a tiny gas leak. Gasses do not come out a wee bit and stop at 50,000ish PSI.
I don't see any pressure signs in any of your pictures. No flat primers. You're not having extraction issues or anything else.
 
Thanks a lot guys for sharing your knowledge!!!

That load was very accurate but a little bit over recommended maximum load with Vihtavuori n140.

Sorry for the swedish word "och" which means "and". Didn't know it was part of the company name. Lapua is from Finland and maybe it means something else over there. If you want to know anything about Sweden feel free to ask, I live very far north but have worked in most parts of the country. Anyway thanks for the answers!
 
When it comes to reading primers, it becomes relative. The brand of primer like CCI vs Federal will make a difference, as will magnum vs standard vs match for certain sizes. For instance magnum small rifle CCI 450 and BR-4 have different cup thickness than CCI 400, and then there is the brand difference.

I make this point because primers with softer and thinner cups will crater or flatten earlier and there may not actually be a pressure issue. On the other hand a hard thick magnum may see a sticky bolt before you notice anything obvious with the primer. So really reading the primer requires you to know what it should look like in you R given situation. I know this sounds confusing but I don't want you to think that the primer is some kind of supreme indicator.
 
This year I built a rifle. I used Nosler brass for 25-06 [SAAMI reamer] and found the pressure limit was loose primer pockets. I backed off 2 grains. I shot a buck mule deer.

This year I also built another rifle and used Lapua brass for 6mmBR [no turn tight neck reamer] and found the pressure limit was pierced primers. I backed off two grains. The Rem700 was hard to cock. I took the bolt apart and it was full of little divots from pierced primers. I put the bolt back together and shot a buck and a doe antelope.
 
Over pressure signs don't show up until you hit about 70,000 psi. Your brass looks fine to me, but you could still be 5000-10,000 psi over loaded and have the cases look fine.

Best way to know for sure is to buy a chronograph. With good models selling for under $100 there is no reason not to buy one.
 
.
jmr40 Over pressure signs don't show up until you hit about 70,000 psi. Your brass looks fine to me, but you could still be 5000-10,000 psi over loaded and have the cases look fine
Click to expand...

There are exceptions to this rule. In my .338 LM, when I first started hand loading for it, I used hornady brass. This was a disappointment as I had to load it 260 fps slower than the published max velocity as the cases got stuck. Stuck case are typically a sign of over pressure. When I switched to lapua brass I could load at or slightly above the hodgdon published max velocity with my 26" barrel with absolutely no pressure signs.

Other .338 LM owners found the same issue as hornady cases in that caliber were too soft.
 
I had to load it 260 fps slower than the published max velocity as the cases got stuck. Stuck case are typically a sign of over pressure. When I switched to lapua brass I could load at or slightly above the hodgdon published max velocity with my 26" barrel with absolutely no pressure signs
Click to expand...

Although that is correct , stuck cases or sticky bolts are an indicator of over pressure . As you showed , You were not over 338 LM pressures or your velocity would have been closer to the intended velocity . You were over the pressure the softer Hornady brass could handle . I recently had the same problem with some 308 LC-14 brass . This specific brass can't be loaded over 41gr to 41.5gr of IMR 4895 pushing a 168gr bullet to 2500fps with out sticky bolts . How ever when using the same components in LC-12-LR brass I can get a 43gr charge and almost 200fps more velocity with no sticky bolts . These cases avg almost the same case volume with the LC-14 avg .3gr less . Not enough to have a 1.5gr to 2gr charge difference .
 
Others have only supported my theory that reading pressure signs in brass/primers is like reading tea leaves. Depends on the brass used, primer used, cartridge you are firing, and other less obvious variables.
 
I know what you are saying Metal God, that I wasn't over chamber pressure as illustrated by my chroney data.

My point was that sometimes tell-tale over pressure signs like sticky bolts or even pierced primers show up when you are still in fact well below the max SAAMI chamber pressure if there exists some other issue.

You should still initially assume it is pressure for safety's sake, but there are instances where over pressure isn't the problem.
 
You should still initially assume it is pressure for safety's sake,
Click to expand...

Agreed 100% . I went round and round with that LC-14 brass until I realized with help from Unclenick it was the brass it self that was the issue . At first I thought it was a primer issue because I had changed primers when I changed to the LC-14 brass . How ever after testing exact duplicate loads in both the LC-14 and LC-12-LR cases I realized that the issue was the softer brass .
 
chronograph

Post #4. What whiskey said. These can help a lot. You do not have to have a max charge to achieve max pressure. You need to look at velocity. To do that, you need a chronograph. There are some good, affordable ones out there.
 
Not just the choronograph, but the chronograph and QuickLOAD. Velocity does not tell you peak pressure. The kinetic energy, which is proportional to the square of velocity, will tell you the average pressure in the bore when a little basic physics of force and acceleration are invoked. But it does not tell you peak pressure, as the average is of all pressures from start to muzzle. This is why chronograph instruction warn you not to use a chronograph to determine safe loads. That, and the fact many optical chronographs can be off by over 100 fps under some lighting conditions, so they have some liability issues there.

That said, you can cheat. If you have reason to believe your chronograph is behaving well*, you can estimate from published velocity and pressure data and barrel length adjustment allowance what ballpark the peak pressure is in. I put this into Excel awhile back. You basically need data like Hodgdon's or Lyman's that has a start load and start pressure and test barrel length for the kind of bullet you are using. Plug it and your barrel length in, and the Excel file estimates velocity and peak pressure combinations you should get from different powder charges. Tweak the powder charge away from reality until a matching velocity is found, and the peak pressure is probably in the ball park. For better precision, barrel length should be taken as the bullet travel distance. That is, subtract the distance from the cartridge head to the bullet base from rifles or pistols or single-shot handgun barrels. For revolvers first add the cylinder (chamber) length to the barrel length, then subtract that same distance. If the testing was in a SAAMI standard compliant barrel, you can look up the actual lengths of chambers made with vents to mimic a barrel/cylinder gap from the standards at the SAAMI web site. The test barrels are listed in roughly the last third of each standard.

You can download the Excel file here for the next 30 days or so: http://www.filedropper.com/pressureandvelocityestimatesfrompublisheddatalockedversion

That version is locked except for the input arguments to prevent accidental alteration of the formulas, but you can see them or do a copy and paste to a blank sheet to have an unlocked copy if you think you know what you are doing. As usual, you use this sort of thing at your own risk!!!!! Neither the author nor The Firing Line make any guarantee of its accuracy, not do they take any responsibility for the reliability of the results you guet, nor for what you decide you can and cannot do based on those results.




*A rifle in .22 LR and some good grade match ammunition that you've tested previously over the same chronograph are a good way to check. This is because the huge expansion ratio in a long barrel .22 LR uses up all the powder's energy and the bullet coasts without much velocity change from about 18" to about 26" of barrel length. Match .22 LR ammunition is generally within 50 fps of its manufacturer's claimed velocity from any barrel in that length range.
 
You must log in or register to reply here.
Back
Top