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  #1  
Old 01-24-2011, 11:29 AM
20ozjolt 20ozjolt is offline
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A friendly reminder regarding "signs of over pressure"




I know we have all heard this before but in an earlier post a member gave load data for the. 45 ... a top end load.... with the statement that there were no signs of over pressure.....

IF in a pistol (other then a :556 or some other" high power" one) you have any signs of over pressure you are way way past the danger point. Hand guns operate at far to low of a pressure to cause them... so please don't use them as a way to tell if your load is safe.

Safe reloading and shooting all.
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  #2  
Old 01-24-2011, 11:31 AM
Cannibul Cannibul is offline
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Some of the reloading manuals are starting to figure this out too.

Even many rifle loads won't show the classic "signs" till you are at or over the danger point.
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  #3  
Old 01-24-2011, 12:34 PM
Nick A Nick A is offline
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The typical "signs"...

...are like the stuck horn on a car after an accident.
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  #4  
Old 01-24-2011, 12:37 PM
Nick A Nick A is offline
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I remember...

my 1968 VW bug.
It had a red light on the dash that illuminated when the brake pedal went to the floor.

That's what pressure signs are all about. The red light that says, "Too late."
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  #5  
Old 01-24-2011, 12:54 PM
Cannibul Cannibul is offline
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Since brass is brass the signs that appear at say 65,000+ psi in a rifle mean you might be in the danger area in the .45 ACP.
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  #6  
Old 01-24-2011, 01:11 PM
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Rifter Rifter is offline
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Quote:
Originally Posted by Nick A View Post
my 1968 VW bug.
It had a red light on the dash that illuminated when the brake pedal went to the floor.

That's what pressure signs are all about. The red light that says, "Too late."
The typical 'signs' most people look for are ejector marks in the case head, torn case rims, primers that flat and smeared, etc. As noted, in a handgun when you start seeing stuff like that, you'd better start counting your fingers and eyes, and I agree that you're well past the 'End of Road' sign well on the way to the washed out bridge ahead.

However, there are some signs that you can use to get a good idea you are walking on the edge of the cliff and the cliff is starting to crumble.

Whenever I started working with a new cartridge, I fired a batch of factory ammo, and took case measurements of the fired cases at the point of maximum expansion on the case wall. That is easy to do, and it gives you a baseline to work from. If your reloads are giving the same measurement at the same point, you know you are roughly the same pressure as factory. Under that, its lower, and above its higher.

On the high side, when expansion begins to exceed the baseline, pay close attention to how hard it is to extract. As pressure increases, the case walls will stick tighter to the chamber walls, and not shrink quite as much, thus it takes more effort to pull them out of the chamber after firing. Subjective recoil will likely increase as well.

If you use a chronograph (and you should), in most cases you'll see a steady rise in velocity increment as the powder charge goes up, ie. X fps more for each .1 grain of powder. When that linear progression flattens out, you should also be seeing greater case wall expansion, stickier extraction, etc. That's when you stop and back off.

It takes some experience to make use of such techniques, and it isn't always precise, but if you're careful and keep good records, those techniques can help you be a better reloader getting the most out of your loads yet still being safe.
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  #7  
Old 01-24-2011, 01:43 PM
Nick A Nick A is offline
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Rifle reloading

Rifter's words also reflect my own real-world experience in modern centerfire rifle reloading. Good advice.

Of course, this does not replace published data. This describes a method to make observations as you work within the published data, just in case the data has a typo or you made a mistake at the press.



Let me add a note, if I may:

According to Speer, typical case head (not the body, but down at the web) expands about 0.0003" to 0.0005" per shot (that's 3 zeroes before the numeral), and about 0.0005" to 0.0007" for a badly overloaded cartridge. Calipers will not distinguish between those dimensions. Most micrometers won't either. You need to get a very, very good micrometer to measure those numbers reliably.
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  #8  
Old 01-24-2011, 01:53 PM
FreeAmerican FreeAmerican is offline
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I have one for you. A 9mm question. I'm going to leave out the brand name of the pistol, would like to see what you think.

9mm lead 124gr using Bullseye. Charge taken right out of my recent book. In this one pistol the dent in the primer seems to be pushing back out a little . No flattened primers, just the dent is pushed back almost flat with the rest of the primer. No hard ejection marks.

Shoot the same rounds in a diff 9mm, no issue??? I was sure it was a pressure sign until it did not do it in other pistols.
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  #9  
Old 01-24-2011, 05:09 PM
TheGerk TheGerk is offline
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Quote:
Originally Posted by Nick A View Post
...are like the stuck horn on a car after an accident.
That was a Great analogy Nick...
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  #10  
Old 01-24-2011, 05:23 PM
TheGerk TheGerk is offline
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Quote:
Originally Posted by FreeAmerican View Post
I have one for you. A 9mm question. I'm going to leave out the brand name of the pistol, would like to see what you think.

9mm lead 124gr using Bullseye. Charge taken right out of my recent book. In this one pistol the dent in the primer seems to be pushing back out a little . No flattened primers, just the dent is pushed back almost flat with the rest of the primer. No hard ejection marks.

Shoot the same rounds in a diff 9mm, no issue??? I was sure it was a pressure sign until it did not do it in other pistols.
Quote:
"just the dent is pushed back almost flat with the rest of the primer. No hard ejection marks."
This could definitely be a “sign” of excessive pressure
Typically the next stage would be primer flow
Since the case head is forced against the breech face on ignition, the only “unsupported” space on the primer is the area of the firing pin hole.
On the breech face the excessive pressure will attempt to “flow” the primer into the firing pin hole.

If you see fired cases with a raised edge in the center of the primer (usually this can be felt as well by your finger) this would be a clear “sign” that something is making excessive pressure with this cartridge, with this loading, in this gun.
Good Luck
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  #11  
Old 01-24-2011, 05:45 PM
Nick A Nick A is offline
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Two thoughts for FreeAmerican...

1. Pistols like CZ (and others) may tend to have short chambers in 9mm. If your lead bullet is loaded long, it may jamb into the rifling lands, which allows more pressure to build until the bullet starts to move. Your other pistols don't do that because the chamber lengths are more generous.

2. Glocks and their kin have square firing pins. Those square strikes sometimes become 'outies' at normal pressure. Nothing bad, just unusual.
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Old 01-24-2011, 07:57 PM
WESHOOT2 WESHOOT2 is offline
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an easy reminder

When your load exceeds even one published maximum load.
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  #13  
Old 01-24-2011, 08:37 PM
Steve4102 Steve4102 is offline
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What do you think caused this? New DW PM-7, 200gr LSWC, WST.
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  #14  
Old 01-24-2011, 08:42 PM
fecmech fecmech is offline
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A weak firing pin spring or possibly a dirty firing pin hole preventing the pin from retracting before the slide moved would be my guess.
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  #15  
Old 01-24-2011, 10:46 PM
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Quote:
Originally Posted by TheGerk View Post
This could definitely be a “sign” of excessive pressure
Typically the next stage would be primer flow
Since the case head is forced against the breech face on ignition, the only “unsupported” space on the primer is the area of the firing pin hole.
On the breech face the excessive pressure will attempt to “flow” the primer into the firing pin hole.

If you see fired cases with a raised edge in the center of the primer (usually this can be felt as well by your finger) this would be a clear “sign” that something is making excessive pressure with this cartridge, with this loading, in this gun.
Good Luck
Had to read your post twice to make sure before I posted this. It might be due to high pressure, and it might not. If the firing pin hole is oversize, OR the firing pin itself is undersized, you can get primer cratering just like you describe but pressure is not too high. I would be more inclined to go that route than with high pressure especially if everything was normal in a couple other pistols of the same caliber.
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  #16  
Old 01-24-2011, 11:42 PM
Atlanta1911 Atlanta1911 is offline
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Quote:
I have one for you. A 9mm question. I'm going to leave out the brand name of the pistol, would like to see what you think.

9mm lead 124gr using Bullseye. Charge taken right out of my recent book.
Mr.Glerk hinted at this, but let's back up and hit the details.

No reloading manual ever told anyone to start in the middle of the range. IMHO this is one of the core issues that gets people in trouble. There are simply too many variables to know what your exact chamber pressures will be. Chamber pressure is the result of powder and OAL, but also barrel roughness, barrel size, type of rifling, bullet hardness, bullet proximity to the rifling, tightness of the chamber, ambient temperature, humidity, etc, etc. So the exact same cartridge must give 2 different pressures in 2 different guns, especially if it wasn't on the same day.

This is why we're given "starting loads" and told to work up from there. There's only 1 way to believe your chamber pressure is the same as the load reported in the manual and that is to use a chronograph and compare bullet speeds. Without a chrono you must assume your pressures are going to be higher.

Anything else is just wishful thinking.
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  #17  
Old 01-25-2011, 07:13 AM
40dcoe 40dcoe is offline
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Originally Posted by Atlanta1911 View Post
This is why we're given "starting loads" and told to work up from there. There's only 1 way to believe your chamber pressure is the same as the load reported in the manual and that is to use a chronograph and compare bullet speeds. Without a chrono you must assume your pressures are going to be higher.

Anything else is just wishful thinking.
Chronographs measure velocity, not pressure. You cannot equate pressure
and velocity. It is wishful thinking.

Joe
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  #18  
Old 01-25-2011, 08:08 AM
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Rifter Rifter is offline
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Chronographs measure velocity, not pressure. You cannot equate pressure
and velocity. It is wishful thinking.

Joe
While they don't measure pressure directly, they can let you know when you are getting near the top.

Every powder I've ever worked with shows a linear progression in how it gains velocity as the charge increases. You can graph it if you wish to. There is a crossover point at which adding powder no longer increases velocity and the line flattens out. At that point, adding more powder only increases pressure, not velocity, and is pointless.

At the same time, you monitor other indicators like casehead diameter and expansion ring diameter, how hard it is to extract, etc. If you are paying attention, you will know when you've reached the point to stop. If you've worked with a particular firearm enough, you can even tell how much you need to reduce charges to stay on the safe side of the line.
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  #19  
Old 01-25-2011, 08:46 AM
Jim Watson Jim Watson is offline
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Steve, a titanium firing pin can give ugly primers with normal loads in some cases.
Does your DW have one?
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  #20  
Old 01-25-2011, 09:13 AM
40dcoe 40dcoe is offline
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Quote:
Originally Posted by Rifter View Post
While they don't measure pressure directly, they can let you know when you are getting near the top.

Every powder I've ever worked with shows a linear progression in how it gains velocity as the charge increases. You can graph it if you wish to. There is a crossover point at which adding powder no longer increases velocity and the line flattens out. At that point, adding more powder only increases pressure, not velocity, and is pointless.

At the same time, you monitor other indicators like casehead diameter and expansion ring diameter, how hard it is to extract, etc. If you are paying attention, you will know when you've reached the point to stop. If you've worked with a particular firearm enough, you can even tell how much you need to reduce charges to stay on the safe side of the line.
Equating efficiency to pressure fails, too. Max efficiency could be thousands
of PSI under max, or thousands of PSI above max.

Chronographs measure velocity. Period.

Joe
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  #21  
Old 01-25-2011, 12:17 PM
Atlanta1911 Atlanta1911 is offline
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Quote:
Chronographs measure velocity, not pressure.
Exactly correct.


Quote:
You cannot equate pressure and velocity.
Also exactly correct.

But no one said you could "equate", but you CAN relate the two using the pressures in the load data. If your chrono says your bullet is traveling at 1100 fps*, and the load data tells you that during tests in a ballistics lab using the same caliber, powder and bullet that 1100 fps was reached at 17,000 psi, then you can relate you findings to the lab's and know you are somewhere close to 17,000 psi. Again, I'm NOT saying you're "exactly" at 17,000 psi, I'm saying you're simply in the vicinity.

And that's as close as you'll come to knowing what your actual chamber pressure is without gauging your gun. Is it "exact"? No way. But neither is reading primers or cartridge cases.

All the best.


* The one caveat to this is that I'm assuming you've worked your load up from the "starting load" and you are mapping your velocities as they climb. If you pull a load out of the middle or upper end of the manual's load range, then all bets are off. As Rifter pointed out, you have no way of telling if the pressure-velocity relationship is still holding true. Another way to say that is "one data point doesn't make a graph".
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