Question: Excessive Pressure Signs?

[Wildernesshunter]

I have reloaded for years, but I have to admit, I don't think I have ever witnessed a excessive pressure failure. I have always adhered too.....staying below the maximum CUP numbers. I have a few questions....

1. What are the signs, besides a gun blowing up, of the pressures getting too high for a particular load?

2. We live in a legal/attorney world, so does everyone agree that manufacturers of guns and actions actually build their parts and guns 10% or 25% or 50% stronger than Max CUP?

3. We all know that some of the old rounds that were originally intended for weak actions or were intended for auto-loading have a lower CUP, so as not to blow up a weak actioned gun. But when a round that was handicapped really low, finds itself in a modern day, bolt action rifle.....can those CUP ratings be exceeded?


I ask, because for the first time in my life, I am going to push the envelope. I have a new Howa bolt action 7.62x39. Widely accepted CUP average on this round is 50,000 according to the SPEER Loading Manual. I can get 30 grains of A2200 in the case with a 150gr SP.....which I am guessing will be about a 52-53k CUP.

I have loaded the following.....

28.0 grains (3 rounds) = CUP ????
29.0 grains (5 rounds) = CUP 48,698
29.5 grains (5 rounds) = CUP ????
30.0 grains (3 rounds) = CUP ????

That first 30 grain load....I may pull the trigger with a string.

I expect for some of you to give me the standard......you're bat-CENSORED- crazy....which is fine, but I would like some intelligent responses and any experience you have had.
........

 

[dahermit]

Signs of high pressure:
1) The primer is so flattened that it fills the primer pocket completely.
2) The case head expands more than .001 inch upon firing.
3) The bolt handle begins to resist being lifted.
4) The case begins sticking in the chamber.
5) The case gets stuck in the chamber, the extractor pulls through the extractor rim (However, on an auto like the Remington 742, stuck cases with the extractor tearing through the extractor rim also indicates a dirty chamber.).

 

[m&p45acp10+1]

I stay with a tested load. I do not go above what is published for a maximum. I will check several sources to get a concusses. Also I have found good loads that were not maximum loads. One of my loads is a published maximum in one manual, and two other sources list loads that go another grain higher. So I worked up to that load.


Excessive pressure signs mean you way over did it. Cratered primers happen when pressure has gone way over what it should have. Next is leaky primers. Then a blown primer.

 

[Jim Watson]

That is a 3.45% increase in powder charge.
According to the old rule of thumb as refined by Vihtavuori, that will get you 2.7% more velocity. But it will cost you 6.9% increase in pressure, 52000.
This assuming you have not gone past what old timers like Elmer Keith called the "balance point" of the powder where pressure vs charge goes nonlinear.

Another old rule of thumb was that when you see ONE of the excess pressure signs listed by dahermit, you should reduce the load by 6%. A lot of people these days will reduce the load from a clear overload by half a grain and think they are safe. Wrong.

 

[HiBC]

First,I think there are so many variables...that to pursue"How can I know how far I can exceed max" via a post on this forum is....questionable.
I can't do a real intelligent job of directly answering your question.
I can give you a couple of things to think about.

Expansion ratio,case capacity,efficience..burn rates. 7.62x39 is a small case compared to the bore dia.
I assume you are not pursuing pressure for pressure sake,you want velocity.
Just spiking pressure is not the same as more velocity.You can spike pressure with very fast handgun powder without gaining velocity.(versus a slower powder)
I don't load 7.62x39.I can't select your powder,but with your small case volume,I think you might gain more velocity choosing the perfect powder over higher pressure.Choose a powder that fills between maybe 97% to 102% of the case volume,and shows top velocities at acceptable pressures.
I think if you choose appropriate powders,you will be limited by case capacity before pressure.
If you were loading for 300 Rem Ultra Mag,you would have a big boiler room ,lots of powder,and a relatively small hole to let the pressure expand/reduce.Its easy to get excessive pressure.Not so with the small case of the 7.62x39,IF you use proper powders.

Point two,burn rates are not linear.Pressure curves are not linear.As the heat and pressure go up,the burn rate,and pressure,accelerate.Its not a perfect analogy,but think in terms of octane,gasoline,and "pinging" or detonation.
As you get up into higher pressures,they get harder to control.Things happen more suddenly.
This next one asks,what are you trying to achieve?
I used to do some wildcatting.I did not have load data.I did not rely on any one tool to estimate safe pressure.But this was one useful tool.My chronograph.
Sierra Bullets used to send a newsletter.They described a technique to help when using modern,high pressure actions.
Load a series of cartridges with an incremental increase in charge.Say 1/2 gr.
You will get approximately a linear change in velocity for a while.Maybe 30 fps for each 1/2 gr,for example.Things are pretty good(maybe) so long as you get that increase without any other signs of pressure.
But then you will make an increase that gives less return on investment.You add the 1/2 gr of powder,and you only get 18 fps.
The powder is spiking pressure without gaining much velocity.You hit the wall.
Back off whatever you think your face is worth for a margin of safety.(Older brass,a hot day,a hotter lot of powder,etc)
And be happy.
OK,one more thing.Once you find "the wall" you get less velocity for the pressure increase.So,that extra 1/2 gr adds a little drama.Maybe a lot,but little gain in true performance.
Now,run your ballistic program.See just exactly what that last 35 fps ,or 70 fps gains you in "max point blank range trajectory" or wind correction or whatever it is you need the velocity for.It won't be a lot.

Congratulations!! I'd like to have one of those little Howas

 

[jmr40]

Reading for pressure signs is about as accurate as trying to predict the future by reading tea leaves. The best way to know is to use a chronograph. Bullet speed is directly related to pressure. If the velocity shown on the chronograph is below what the book says is the expected speed with a max load then you are below max pressure.

But you do have to account for barrel length. If a max load for my 308 is supposed to be 2900 fps from a 24" barrel and I'm getting 2900 fps from a 20" barrel it is probably a little over pressure. Even if no traditional signs show up. Those don't become apparent until you hit 70,000 psi in most rifles which is well over max.

Reloading without a chronograph is like driving a car with no speedometer. Waiting for pressure signs to show you you're over pressure is like having a warning light in your car that only comes on when you get over 100 mph, but no speedometer. Having a chronograph lets you know that you are approaching max loads before you get there instead of waiting until you are well over when pressure signs show up.

 

[Wildernesshunter]

Alright....had a great day at the range and was very happy to be there away from the election hysteria.

By the way, I have a 20" barrel.

In order to get the AVG for each group, I threw out the high and the low and then averaged the remaining. High fps is the very highest that was recorded, but not a part of the AVG, as it was thrown out.

Here's the Chrono Data:

Grains...........Avg fps.............High fps

28................2,176.................2,224
29................2,244.................2,264
29.5.............2,280.................2,299
30................2,287.................2,326


There was nothing freaky that happened and none of the cases were overly tough to get out of the barrel. All the primers were a little flattened, but the 30gr were flatter than the 28gr. But the 30gr were no more flattened than the primers on my 325WSM that I load below CUP.

Interestingly, accuracy seemed the best with 30gr of powder, but that is a tough one, as I only had five rounds per group....and then there is the whole human error part.

I've got 24 pieces of once fired brass and I will load them all up with 30gr of A2200 and go back to the range. This time, I will shoot at the 200 yard markers and really check the accuracy.

Just my gut, but I am not comfortable pushing beyond 30gr.

 

[Dufus]

Bullet speed is directly related to pressure.

Don't wanna start an argument, but this is not necessarily true.

Here is a good read on identifying pressure signs.

http://bulletin.accurateshooter.com/2011/04/evaluating-pressure-signs-in-reloaded-cartridge-brass/

 

[Clark]

I have experimented to see how much power is possible with the 7.62x39mm.
I have analyzed the Howa 1500 action.
After my last post, I guess you will have to pm me.

 

[Longshot4]

What is the purpose of pushing the envelope? Isn't the bullet that needs to perform for a clean kill. Also isn't the accuracy that puts the bullet where it belongs. Flat primers are hot loads and when you see them most likely the accuracy load will be back a few grains.

Have some one take pictures of just the rifle after it blows up. Have them wipe off your blood and send us pictures so we can tell our fellow loaders the story.

 

[Wildernesshunter]

<<What is the purpose of pushing the envelope....>>


That's a fair question. One I am asking myself for the past couple of hours. Why push the envelope? Well, I would like to have 1,000 ftlbs of energy at 300 yards with a 150gr bullet.

Let's look at the numbers on the 29gr load with my Chrono data....

29.0gr - 300yds
FPS..........Energy
1796........1075


....and on the 30 gr load.

30.0gr - 300yds
FPS..........Energy
1834........1120


This is data at 6,000 feet elevation.

Maybe I am just fine with 29.0 grains, which is under CUP maximum.


........

 

[Longshot4]

What is your bullet choice? If you use a quality bullet for hunting deer you will only need to hit your target. You will have plenty of energy.

 

[Wildernesshunter]

Its a Speer 150gr .311 Soft Point.

It will hit 2-3 deer in the next two weeks.....I'll recover the bullet and post pics.

Others have shared with me that this bullet might not expand as desired at 300 yards. I'll just have to see on a 'real hide test'.

......

 

[HiBC]

Seems like you are finding your path.

You show 62 fps difference,29 to 30 gr.
You show 40 fps difference 28 to 29 gr.

That is backward from the pattern I ordinarily see approaching max.
I do not doubt your experience,but I cannot read it as I usually would.And,there are not enough data points to really see a trend.

Evening chrono reads can be fickle due to lengthening shadows.Or? who knows.
Jumping to conclusions,you show 50% more velocity increase for 1 gr powder charge.But I don't know we can say it means anything.

Reading primer flattening is over rated.Don't ignore it,but don't draw conclusions.Take it in as a factor.

Did the bolt lift easy in all cases?

Your innards are telling you 30 might seem a touch hot? Did I read that right?.
29 is a book load?

Is 62 fps going to make the bullet perform vs not? Noticable trajectory difference? Deader deer?

Hmmm.

Do you think you might get a little better brass life at 29?

I'm guessing you don't have a lot of range time before season.Got any other priorities?
Good luck!!

If I had to guess,at modest velocities,with a 150 gr,a Ballistic Tip or Hornady SST might ,I say MIGHT,give reliable lower vel expansion.

Just a guess,but something about the 2000 fps zone seems to influence cavitation and bullet performance.Ballpark.

I might look more at that 2000 fps zone,+or-,more than pure ft lbs energy to help determine max range.Of course,both matter.....in shades of grey

 

[Stats Shooter]

Some folks on here may not be as comfortable as I am pushing the envelope, but I often do ladder tests that exceed published maximums. My goal is always best accuracy and often this is well below maximum but with slow powders it seems to be near or above the max sometimes. A chronograph helps but only to a point.
If you are doing a ladder, and planning to test hotter loads, ensure that you are moving up in very small increments.
If you are moving up in very small increments, say 1/2% of maximum, it is highly unlikely you will have an unsafe situation before you recognize pressure signs. Here are my pressure signs I look for. The order may change on what you notice first depending on primer hardness, brass quality etc.
1: Flat/flowing primers where they begin to press outward into the pocket
2: Extraction marks on the case
3: extraction problems like sticky bolt or failure to extract in a semi auto
4: primers actually having the primer firing pin dent protruding outward or loose primer
5: pierced primer
6: bulged or stuck case.

If I notice any of these i stop, pull any remaining bullets and discontinue

Also, keep in mind that if you operate near max and it is colder when you develop the load, on a hot day you may wind up well over tolerances. Same if you do all testing cold barrel and then let things get hot

 

[Clark]

Mississippi
Here are my pressure signs I look for.

There is a big difference between a Ruger #1 and a Rem700 in a work up.
The Rem700 has an ejector plunger hole in the bolt head where brass can flow and then get sheared or swiped off when the bolt turns.

 

[F. Guffey]

2) The case head expands more than .001 inch upon firing.

dahermit, as I read through this thread I am reminded of the blind men and the elephant. If I fired a case once and the case head expanded .001" I would think the case head expanded 4x more than it should have and I would be convinced I was practicing bad habits. Normal, factory loaded, over the counter ammo should only expand .00025". When measuring case head expansion the target case head expansion is .00025".

And then there is the 'and then' thing. If the case head expands it is not only the case head that expands but the flash hole expands and the primer pocket expands and the case head crushes. reloaders are under the impression 'it happens suddenly and all at once and without warning'. And then there is the case head imprinting on the bolt face. If the bolt face is not smooth and level expect case head imprinting and then there are case heads that are soft, how soft? Who knows? I like case heads that expand a little, I would choose a case head that was a little soft over a case head that is brittle. But; reloaders are so infatuated with getting 50 firings our of a case and I always ask; "What did the case weight when you started and what did the case weight after firing 50 times?".

F. Guffey

 

[Unclenick]

Wilderness Hunter,

I keep a list of pressure signs.

As jmr40 said, this can be like reading tea leaves. The best approach is to look for multiple pressure signs. Primers and brass all start moving at different pressures and are not calibrated to a specific pressure, so what you see on them only lets you conclude a shot did or didn't exceed what the primer or the brass were happy with, but it's not as absolute as a pressure value. If you want a good way to try read pressure, get one of these.

Normally you get a fairly linear increase in velocity with powder charge. Failing to get that linear increase with powder charge is a pressure sign, as it suggests the chamber space got bigger (steel is stretching). Accuracy tends to deteriorate at that pressure, too. From your list of averages, you had an increase of 68 fps/grain from 28 to 29 grains, of 72 fps/grain from 29 to 29.5, (which I would call the same as 68 within the margin of normal average variation error; the standard error), and a measly 14 fps/grain, from 29.5 to 30. That is a pressure sign if your sample was adequately large to reveal such things. If it wasn't, then it could just be random and you should repeat the experiment.

 

[TheGuyOfSouthamerica]

Visual pressure signs

Hi,

Here my Nagant type reloads shot today in an Pietta 1873 SAA 357 mag Revolver 5.5". The bullet is an Lee 148 grain Wadcutter (Elmer Keith style grease grooves) seated 0.113" deeper than flush (hence "Nagant style") in an 38 spl case. The empty case Volumen is about the same as is in an 9mm Luger case. I use WSP primers.

Here the photo

...
Bottom: is an loaded round. Observe the nice round primer.
Second Bottom: 124 grain Lee .356" Truncated Cone bullet. No sign of pressure.
Third&Fourth Bottom: 148 grain Lee Wadcutter. 3.4 grain VV N340 type powder (it is actually Vectan A1). Signs of pressure. Flattened primer and a Little cratered firing pin hole.
Third Top: Same 148 grain bullet. No sign of pressure.
Second Top: Same 148 grain bullet. Sign of pressure.
Top: Loaded round with 148 grain WC from top view.

All with signs of pressure will bulge when put 0.1 grain more powder so those I backed up to 3.4 grains of VV N340 (it is actually Vectan A1 what I use).
None of the pressure sign cases were sticky but one at the Point to be sticky.

First pressure sign in an Revolver is primer flattening (like in the Picture), then firing pin hole cratering and then sticky hard to extract cases.

 

[Clark]

Wildernesshunter
That first 30 grain load....I may pull the trigger with a string.

My father built a gun for himself in the 1940s and tested it with a trigger string.
I was born in 1951.
In 1999 I started handloading.
In 2000 I was doing all kinds of tests involving trigger strings.
By 2001 I knew which way parts would fly if a gun or case failed, was more careful about working up to a failure, and so stopped using trigger strings.

But a few of those trigger string experiments made lasting memories:
1) S&W 60 38 special not attached well enough to fixture. I was hiding behind a concrete wall. The recoil caused the revolver to come flying over the wall, over me, and keep going.
2) Kel-Tec P11 9mm underground in sheet metal box. The gas caused that box to expand, then shrink inward, breathing sandy dirt into every crevice inside the pistol.