Question about "Over Pressure"

greg11

greg11

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So, I understand there is a SAMMI Spec regarding loads, and that some action manufacturers are known for designing "stronger" actions than others (meaning you can push the limits a little). I don't intend to do this, but in some research on working up loads, I came across a guy discussing a load he was trying.

This guy was working up a .223 load and he started at a low charge weight and began progressively working up. As he got to what he thought should be the upper limit of what was possible with his load with respect to pressure, he began noting "This load showed no sign of over pressure... That load showed a slight sign of over pressure, ect..."

How does over pressure manifest itself when you are reaching the upper limit of a cartridge's (and actions) capabilities? Is it a sound? A feeling? How do you tell?

(Disclaimer):
I know about loading manuals. Please don't reply with "stick to what the loading manuals say". I know experimenting with loads can be dangerous. I'm of course not going to stuff a million grains of powder into a .223 cart and "try 'er out". I'd like to know for my general knowledge.

Thanks guys.
 
All of these methods you can google and in 5 minutes find gobs of information and pictures so I won't post them all here.

The bolt can feel a bit more "sticky" to open after firing.

The primers tend to dish out and fill the chamfer of the primer pocket. This is not completely reliable as some primers are softer than others and what may look like a high pressure is really just a soft/thinner primer material. Federal primers have a tendency to do this, I believe they are softer brass.

Increased cratering of the primer where the firing pin strikes, same caveat as above regarding the primer hardness

You can measure the case diameter before and after, just above the head and look for plastic deformation. You need a very accurate micrometer for this as you will be looking for changes in the 0.0001 range. I tried doing this but not much success, inconclusive.

Possible gas blow-by around the primer, I've never seen this but that's not to say it couldn't happen.

Given the subjective nature of "reading" or "divining" the signs of over pressure, I've never trusted my abilities to gauge what is over pressure and what is not. I can tell it's a hot load by recoil and that's about as far as it goes, I have no idea about what the pressure is. IMHO I don't think anybody could tell you with any certainty what the pressure in a cartridge is by looking at it or measuring it's deformed dimensions, they are just fooling themselves.

I ended up purchasing a strain gauge based measuring system as I wanted something more concrete and wanted a better understanding of what was going on with the whole process. It's very reliable and the science it is well proven.

Check out this link. Good information even if you don't go this route,

http://www.shootingsoftware.com/pressure.htm
 
There are several telltale signs of over pressure.
You will start to see flattening of the primer where the gently round edges start to become squarer. You may also possibly see some backing out of the primer from the primer pocket. Your may also see dimpling around the firing pin mark on the primer where the primer metal is actually raised around the firing pin strike. Other indicators may be reduced case life due to incipient case separation or cracking.
There are several good threads out there if you use the search function and go advanced so you can search the last 18 months or so.
 
Most of the"traditional indicators" have already been noted in the previous posts - with one exception - velocity (In my mind, this is a traditional indicator in that John Barsness has been mentioning it for years, now).

If the velocity exceeds published velocities for a given bullet and weight (assuming you are using the same components as used in generating the data and same barrel length), then you are almost certainly over safe pressures - even in the absence of other inidicators (i.e., those mentioned above).

One interesting assertion recently made in another forum by John Barsness (regular contributer to a variety of rifle magazines, including "Handloader") is that IF you have reached the max charge published in a manual for a given bullet, but have not matched the max velocity AND no pressure indicators have been observed(e.g., ejector marks, stiff bolt lift, etc), it is probably safe to exceed the max charge listed in the manual (but again, NEVER exceed the velocity). If you are interested, the topic appeared in the 24hourcampfire site within the last month in the "Ask the Gunwriters" forum.
 
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BearClan said:
Most of the"traditional indicators" have already been noted in the previous posts - with one exception - velocity
Click to expand...

Good point. I always use a chronograph at the same time as capturing the pressure curve. The velocity data helps to validate the pressure measurement.

There can be instances where an increase in powder charge does not increase the velocity (or not much of an increase). You do not want to go past this point as the pressure rise rate can be extreme. If you plot velocity vs powder charge, the line will start off with a steady slope and then have a decreasing slope and finally plateau with a slope=0. The area with decreasing slope is where the pressure starts to climb at a more rapid rate and overpressure can occur.
 
Here is the John Barness reply about pressures I referred to in my prevous post:

John Barsness
"The rule of thumb between pressure and velocity is that pressure rises at twice the rate of velocity.

The major point, though, is that if a loading manual states that a 165-grain Nosler Partition and 58.0 grain of H4350 produces 2800 fps in 24" test barrel, then if 58.0 grains only gets 2700 fps in our 24" barrel, the pressure produced is lower than in the test barrel.

This is common in factory rifles, which often have looser bores than test barrels. If we increase the powder charge so that our rifle also gets 2800 fps, then the load should still be safe in our rifle."

The exception is when our rifle already shows definite signs of bigh pressure with 58.0 grains of H4350. This shouldn't happen, and in fact in my experience almost never does, partly because factory barrels (or even custom barrels) almost never tighter than test barrels. But once in a while a "hot" lot of powder comes along, or we end up with a thick batch of brass, or substitute a much hotter primer than the one used in the loading manual.

Or we can be getting false "pressure signs" due to various factors, including soft brass--and soft brass seems to be occuring more frequently these days. I'm not sure why, but I've run into some, as have other handloaders I trust.

This is exactly why a chronograph is so useful in working up loads. If our rifle gets MORE velocity than the manual suggests with a certain bullet/powder combination, then pressures are higher. If our rifle gets LESS velocity, then pressures are lower.

I'm not saying that we should absiolutely rely on the chronograph when working up loads, but it is yet another tool. It's als a much more accurate tool than measuring case heads or examining the flatness of primers.

I know this because I have access to more than one pressure lab, and have run a number of experiments using cartridge head expansion, chronographing and "traditional" signs of high pressure such as primer flatness, bolt lift, etc. By far the most accurate tool is the chronograph.

In fact, before there was ANY published handloading data for the .300 WSM, I was assigned to work up some data in a Winchester Model 70 and do an article for HANDLOADER. Instead of measuring case-heads, or backing off a grain or two from loads that caused stiiff bolt-lift, I calculated how much velocity the .300 WSM should get by comparing it's case capacity to the .300 Winchester Magnum. (This is easy with the 1/4 Rule.) It turned out the .300 WSM should get about 97.5% of the velocity of the .300 Winchester--AT THE SAME PRESSURE.

Then I worked up loads, using the same bullets and powders listed in various manuals for the .300 Winchester Magnum. When the powder charges reached the predicted velocity, I stopped. Within the next year a bunch of pressure-tested data for the .300 WSM came out. Over half of my "chronograph predicted" loads EXACTLY matched the published data, and the rest were within 1/2 to 1 grain.

The big trick, however, is to use data for EXACTLY the same bullet. Many handloaders still make the mistake of thinking that data for any 130-grain .270 bullet can be used with any other 130-grain .270 bullet. That assumption is not only not true but can be dangerous"
 
Junkdude in post #2 has it. Strain gauge. Period. Boltonscouter, on the other hand, inpost #3, perpetuates the now blown-all -to-Hades myth that any or all or some of the the physical manifestations will show themselves before the gun simply comes apart. Which in itself is a fairly definitive indicator, but since I am not paid for that kind of thing, an indicator I choose to avoid.

Strain gauge, chornograph, max book vel for bullet weight and cartridge, always stopping at the first negative indicator. Or, welcome to the world of bragging rights photos in gunshops all over America/Canada. Yup, those boys knew what they were doing too!
 
my tikka left extractor marks, from a hot test load, was just a dimple on base of case. with no other pressure signs eg: chrony kept climbing, primers kept seated, bolt wasn't stiff.

but i use a boatload of extra precautions when trying these loads
 
Apologies for delivering a less than direct answer. You were clear about this, however;

For anyone wishing to go down the road less traveled, I'd recommend they look into getting a copy of a software called "Quickload". Its not a substitute for observing pressure signs but its certainly better than going into no man's land completely blind and it gives you expected peak and residual pressures , in chart form and empirically, for any load you care to input.

You first run comparisons with safe loads you've crony'd, to establish whether the software results track high, low, or spot on with your range velocity results. You can then predict pressures using your software and develop cartridges using predicted pressure increments instead of, "suck it and see", powder increments.

The software can take some getting used to but the more you use it the more you'll use it.
 
Don't let Stubblejumper see you mention the use of a chronograph as a way to gauge pressure, or you'll have a 20 page discussion on your hands! :D

Just ribbin' ya, Stubble...

BTW, a strain gauge is the best method available to the hobbyist handloader.
 
One other validation after the fact - if your primer pockets stay tight for at least 5 repeat loadings, your pressure is not too high. After working up to maximum book load with no other pressure signs, I routinely take 3 cases, load them over and over until the primer pockets are loose. or until I reach 10 loads. Just a confirmation that is simple to do but a little time consuming. It helps to have a hand press at the range.
 
ykkid said:
Junkdude in post #2 has it. Strain gauge. Period. Boltonscouter, on the other hand, inpost #3, perpetuates the now blown-all -to-Hades myth that any or all or some of the the physical manifestations will show themselves before the gun simply comes apart. Which in itself is a fairly definitive indicator, but since I am not paid for that kind of thing, an indicator I choose to avoid.

Strain gauge, chornograph, max book vel for bullet weight and cartridge, always stopping at the first negative indicator. Or, welcome to the world of bragging rights photos in gunshops all over America/Canada. Yup, those boys knew what they were doing too!
Click to expand...

Hey ykkid.......I am not perpetuating any blown all to Hades myth!
The OP asked......... "How does over pressure manifest itself when you are reaching the upper limit of a cartridge's (and actions) capabilities? Is it a sound? A feeling? How do you tell?"
The characteristics I mentioned are legitimate phenomenon. They were in no way intended to be the be all & end all indicators but they are simply things to watch for at the range that require no extra equipment etc. You will note I say "MAY" show these signs........ but that doesn't mean it will necessarily show those sign prior to failure. Just some things for the OP to watch for..... that's all! To conclude that I would advocate that these signs will necessarily manifest themselves prior to catastrophic failure of a firearm is reading way too much into my post! Take it for what it says, nothing more and don't try putting words in my mouth. The visual indicators are merely that and are only one of many useful tools in avoiding an ugly situation, they are simple, free, fast & require nothing but a little careful observation to some details. Strain gauges, chronographs etc are great but they're just additional tools to keeping your eyes open & your wits about you! Why wouldn't you use every trick/tool at your disposal?
 
I just had this same situtation in a 22-250:

The bolt is sticky (very)
Your primers are no longer part of the cartridge or the are flatened right out against your bolt face.
The base of the cartridge has a mirror image imprint of your bolt face on it
Your primer pocket will no longer hold a primer, and there will be visibile signs of gas leakage around the circumference of the pocket.

And in the case of my Savage:

The ejector is stuck buried in the bolt head
Huntsman drills it out for me
Brownells gets $20 for a Savage Ejector / ejector spring / ejector retaining pin
I pull the other bullets I loaded, and check the powder charge level.
I never take for granted that the load recipes on the Varget bottle are scientic, or absolute.
 
Another consideration - older cartridges have a lower design pressure. eg 303 and 30-30 are lower than 308. If you are using visual signs for pressure, then what may be OK for 308 may not be OK for grand-dads 303....
 
There are many things. But as you mentioned below, there can be sounds and feelings:

Sound - A large BOOM as your barrel explodes.
Feeling - Lots of pain due to the result of barrel shrapnel tearing away at the flesh and tissue of your body.

If this happens, you should start reducing your loads.

greg11 said:
How does over pressure manifest itself when you are reaching the upper limit of a cartridge's (and actions) capabilities? Is it a sound? A feeling? How do you tell?
Click to expand...
 
Reminds me of a couple of years back - me and buddy were shooting my new Tikka T3 HB in 6.5x55, all was going well and nice groups were being posted when (during his turn fortunately :D) 'kaboom' - recoil like a 7mm RM, primer stuck to bolt face and bullet hole 8" higher than the previous group. Yours truly must have double charged ! Thankfully no one hurt, my ego bruised, buddy's shoulder sore and we both agreed that Tikka make some super strong actions !
 
Tomochan said:
Reminds me of a couple of years back - me and buddy were shooting my new Tikka T3 HB in 6.5x55, all was going well and nice groups were being posted when (during his turn fortunately :D) 'kaboom' - recoil like a 7mm RM, primer stuck to bolt face and bullet hole 8" higher than the previous group. Yours truly must have double charged ! Thankfully no one hurt, my ego bruised, buddy's shoulder sore and we both agreed that Tikka make some super strong actions !
Click to expand...

that why I weigh my brass and bullets and uses identical weighted componenets .then weigh all assembled rounds before firing ,all weights should be the same.
 
Jordan Smith said:
That depends on the hardness of the individual lot and brand of brass. Some soft brass expands more than other brass at lower pressures...
Click to expand...

Yes, but if your brass does not allow 5 reloads without loose primer pockets, your load is higher pressure than what that brass can support!
Prematurely loose primer pockets = extreme pressure (for those cases) every time!
 
Some guns are stronger than others but the case is the weak link. When it lets go, it can get messy.

I have fired hundreds of thousdands of rounds through piezoelectric pressure guns, copper and lead crusher guns and even a strain guage on my Remington 7400.

By the time you get a sticky bolt or a shiny ejector mark you are probably at or beyond proof round pressures.

There are significant variations of "identical" barrels and chambers and even bigger variations between different source barrels. The laod developed in your buddy's rifle means nothing.

But ther is no free lunch with velocity. If the book says max is 2700 with 4895, then you had better stop at 2700 with 4895.
 
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