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Grizzly
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Double Rifle Pressure Recommendations
      #60592 - 31/07/06 07:43 AM

In looking for some guidance on pressure for DR's, I looked in "Shooting the British Double Rifle". I think on about page 115, he discusses guidelines but they are in tons per square inch. First question is whether he was using the 2000 pound ton or the metric ton (2200#). I am trying to convert it into PSI.

Looks to me like the standards Graehme is using would fall under 30,000 PSI. Is this realistic or too low for modern doubles?

I was corresponding with Norma powders a while back on their published MRP-2 loads for the 470 NE. The max load of 128 grains (15% compression) was off the chart on my ballistics program. Norma measures their pressure in mega paxels, but after a kind soul converted their readings it was over 36,000 PSI. Even their reduced recommendation of 113 grains (2150 fps) ran at between 32-34,000 PSI.

Now I would love to find a powder that fills the case and gets the right velocity. But I can find little, outside of the British Double book, to provide any guidance on pressure.

Thanks



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400NitroExpress
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Re: Double Rifle Pressure Recommendations [Re: Grizzly]
      #60598 - 31/07/06 09:42 AM

Jim:

You're having trouble finding pressure information elsewhere because little else is available.

As for the conversions that you asked about, it depends on what you're trying to convert, and the terminology in Wright's book is a little confusing. I won't say that it's wrong, just confusing.

Working from memory (to be honest, I found his book so useful that I gave my copy away), I think he refers to the original pressure standard for the Kynoch Cordite .470 load as 14 TPSI. That isn't really correct, because it's too easily confused with a measurement of something completely different. The original standard for the .470 was 14 tons BaseCUP (Base Copper Units of Pressure), which was a measurement of bolt thrust expressed in long tons at 2240 pounds to the ton. This value is a measurement of the force exerted on the breech face by the cartridge head as measured in a Base Copper Crusher pressure gun. This isn't chamber pressure, nor does it readily convert to chamber pressure. I seem to recall that some or all of the pressures for the handloads in his book were were also measured in base crusher guns at Kynoch and the Birmingham Proof House, which is probably why your conversions to chamber pressure with these figures seem off.

Chamber pressure is measured radially (on the chamber sidewall) via copper crusher (expressed as CUP, as opposed to BaseCUP) or piezo electric transducer (expressed as PSI or TPSI). The Norma pressure figure that you gave was converted to PSI, which is chamber pressure. The current CIP standard (max average) chamber pressure for the .470 is 2700 BAR or 39,160 PSI. So, yeah, Norma's load is within the pressure spec, but the velocity is too high.
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Marrakai
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Re: Double Rifle Pressure Recommendations [Re: 400NitroExpress]
      #60614 - 31/07/06 06:24 PM

In reply to:

terminology in Wright's book is a little confusing


The 'tons per sq inch' is quoted directly from the ammunition manufacturers' catalogues of the period. The only way to compare modern reloads with the originals is to use the same methodology (even the same equipment in Wrightie's case!).

No-one should really care what the actual pressure is, because that is totally irrelevant. What is critical is that the pressure is equal to or below the working pressure for which the gun was originally proved. Wright set out to deliberately demonstrate this with modern powders and loads. His book is still the best reference for that level of information by far.

There are 2,240 lbs in a ton. A 10-ton load is 22,400 psi.

Keep it simple.....

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hoppdoc
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Re: Double Rifle Pressure Recommendations [Re: Marrakai]
      #60662 - 01/08/06 09:38 PM

Brain fart--

What is the relationship between PSI and CUP again?

Are present day pressure apparati measurements obtained in PSI?

May I assume that most safe Double pressures( not proofing) max around 10 tons ?

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400NitroExpress
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Re: Double Rifle Pressure Recommendations [Re: hoppdoc]
      #60676 - 02/08/06 12:58 AM

hoppdoc:

As a rough rule of thumb only, CUP is usually lower than PSI, but they're not really convertible because the relationship isn't linear. Krieghoff gives 39,885 CUP and 44,962 PSI for their .500/.416. SAAMI lists 35,000 CUP for the .470 and CIP lists 39,160 PSI.

Yes, almost all modern pressure gear is now piezo electric.

Ten tons is BPE pressure so, no, that wouldn't be a good assumption. All of the nitros will be higher.

As this string illustrates, one of the problems with Wright's book is that the pressure data doesn't correlate to chamber pressure (PSI), which is what is used virtually everywhere else today. His use of the term "TPSI" to describe BaseCUP is misleading, as this is easily confused with piezo chamber pressure, and more than a few have made that mistake. "TPSI" was never used in the original data. Pressure was simply expressed as "X tons". At a minimum, with respect to the pressure testing he did at Kynoch, both BaseCUP and PSI were available, as their guns measure both simultaneously. Inclusion of the piezo data would have been of enormous value to handloaders. Also, Kynoch has done extensive testing of Cordite ammo in recent years, so there was no need to limit comparison to BaseCUP only. The suggestion that actual pressures are irrelevant, especially with regard to double rifle cartridges, is patently absurd.
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Double_Trouble
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Re: Double Rifle Pressure Recommendations [Re: 400NitroExpress]
      #60679 - 02/08/06 01:41 AM

400 NE
is your background in engineering ?

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Rusty
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Re: Double Rifle Pressure Recommendations [Re: Double_Trouble]
      #60694 - 02/08/06 10:23 AM

No, he is not an engineer. He is however one of the most well read and studied student of double rifles and their history in the USA today.

OH, he's also one of my best friends! Then the DRSS gets together, I like to stand close to 400 so I look smart!

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4seventy
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Re: Double Rifle Pressure Recommendations [Re: 400NitroExpress]
      #60695 - 02/08/06 11:25 AM

In reply to:

"TPSI" was never used in the original data. Pressure was simply expressed as "X tons".



Huh??????
400, don't go betting your gun collection on that one mate!


Yeah, pressure may well have been sometimes expressed as "Tons" in the "original data" but that was definately referring to "Tons to the Square inch" or "Tons per Square Inch" or "TPSI".
"Tons" was used as an abbreviation.

"X tons" is a measurement of mass or weight, not pressure.



Edited by 4seventy (02/08/06 03:06 PM)

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Marrakai
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Re: Double Rifle Pressure Recommendations [Re: 400NitroExpress]
      #60710 - 02/08/06 06:23 PM

Rusty:
I appreciate the fine credentials of your friend, but with respect, I have never known the actual pressure of a single cartridge I ever fired in my entire life, and that is quite a few hundred thousand counting shotshells and target pistol loads. Several thousand of them were in British double rifles, all handloaded by me.

I know the pressures have always been safe, since I've never damaged a gun.
With reloads I control the quantity and burning-rate of the powder, and can measure the velocity from time to time. This I compare with recommended loads proven safe by comparison with the charges for which the guns were originally proved. Wright's book is a good source of pressure data, undoubtedly the best published reference to date.

Can anyone here tell me the actual pressure of the last cartridge they fired in a double gun? Of course not!

Perhaps it is irrelevant after all.................


And as for the problems understanding Graeme Wright's book, blimey! Couldn't be much simpler. A lot of 'mole-hill mountaineers' might see the need to criticise that marvellous reference, but I don't see anything even remotely near it coming from the quill of any of its detractors.

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Rusty
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Re: Double Rifle Pressure Recommendations [Re: Marrakai]
      #60718 - 02/08/06 11:02 PM

Marrakai,

So how would we translate the bolt thrust pressures of the old Kynoch to PSI Chamber pressures?

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unspellable
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Re: Double Rifle Pressure Recommendations [Re: Rusty]
      #60730 - 03/08/06 07:16 AM

It's even messier than laid out above. Attempts at conversion are futile. While the units used may be coverted easily enough, the real problem is that use of different units implies different methods of measurement. So we have the traditional British method which measures backthrust with a crusher, we have the US SAAMI system which measures chamber pressure with either a crusher or pizo transducer, and we have the European CIP system which measures not chamber pressure but throat pressure with either a crusher or transducer. Five different methods that will yield five different results on one single load. Now throw in the usual measurement errors.

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Marrakai
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Re: Double Rifle Pressure Recommendations [Re: Rusty]
      #60735 - 03/08/06 09:07 AM

Unspellable:
My point exactly!

Rusty:
I would ask the question "Why would you need to?"
The single most important line of text in Graeme's book IMHO is where he states that, if you use medium-slow modern nitro powder and don't exceed the old cordite published velocity, the load should be safe, ie below the original cordite pressure for which the gun was proved.

Personally, I accept that statement as absolute fact, given a sensible approach to fillers if required, and correct bullet weight and diameter. In Australia we also have the luxury of ADI powders, something we generously share with our American buddies through Hodgdon's 'Extreme' range.

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Grizzly
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Re: Double Rifle Pressure Recommendations [Re: Marrakai]
      #60739 - 03/08/06 10:26 AM

The reason I wanted to know was to have some set maximum in mind, especially when playing around with different powder, bullet and even primer combinations.

The GS bullets for example, will give more velocity with the same powder load than with non banded bullets. With a claim from GS of less pressure.

I can get the PSI information for most other calibers and rifles, but not for nitro rounds.

What got me thinking about this was when I looked at Norma's reloading guide for their recommendation for the use of MRP-2 powder in a 470 NE using 128 grains of powder with a velocity of a little over 2200 fps with 26" barrels.

I have a program called quick load, and when you plug the Norma data in you get red warnings all over the place - 15% compacted charge, serious over pressure, warning warning.

Talking to Norma, their charge produced a little over 36,000 PSI. In a bolt rifle, that is nothing. But without any baseline data on a nitro round, that is a number in a vacuum.

I expect that more manufacturers will either start publishing or provide on request the PSI information for rounds, if for no other reason than their damn lawyers will probably make them do it.

So can a double handle 20,000, 30,000, 40,000 PSI or higher? When I have a program that screams overpressure, but someone like Norma saying 36,000 PSI is safe, which way do you turn?

It would be nice to use a slow powder that fills up the case and is accurate. You don't find out the accuracy part until you get to the range. It would be nice to have some guideline to avoid blowing up a nice double.



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hoppdoc
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Re: Double Rifle Pressure Recommendations [Re: Grizzly]
      #60741 - 03/08/06 10:48 AM

I claim ignorance with Doubles but you must be careful with older bolt rifles in assuming that the same velocity equals the same pressure. Obviously this is related to the specific burn qualities of the powder but you must quantify.

Had a bud freeze the bolt of an older rifle with a different powder when the chrono gave the same velocity.

I would suggest that even in Double it would be nice to know a max load(a psi measurement for uniformity) to avoid when trying to regulate for various bullet/powder combos.

What pressures can a modern Double tolerate regularly and still regulate shots properly?

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Rusty
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Re: Double Rifle Pressure Recommendations [Re: hoppdoc]
      #60746 - 03/08/06 11:25 AM

Unspellable is most correct.

Why do you need to know?

Because it is part of being a double rifle owner! I need to know as much as I can garner about old British Double Rifles.
It is not esoteric, nor is it benign. People can make a disastrous decision based on a correlation between fact that cannot be made. That results could at best damage a fine old double or worst damage the owner.

Just because you don't need to know doesn't keep you from potential harm.

The double is a differant animal. While you are waiting for traditional bolt rifle pressure signs to appear, you can already be damaging your double taking unneeded risk.
Just my opine here.

I suggest you go do what you need to do and be happy!



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unspellable
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Re: Double Rifle Pressure Recommendations [Re: Grizzly]
      #60790 - 04/08/06 07:13 AM

The question is not how much pressure will double rifles take, but rather how much pressure will YOUR double rifle take? Doubles have beeen built to handle pressures all over the map just as bolt actions have been. There are doubles that take the 458 Winchester or the 375 H&H. There are bolt action rifles that would be converted into hand grenades at those pressures. A given cartridge will run the same pressure in double or bolt rifle.

Cartridges designed specifically for singles or doubles ran rather low pressures for the simple reason there was no need for high pressures. There is no constraint on case length as there is with a bolt action. The 458 Winchester is an outstanding example of a cartridge running high pressure because of length constraints.

Today we have doubles chambered for high pressure cartridges like the 458 Winchester, not because the high pressure is really needed but because of the ready availability of the 458 Winchester cartridge. A 450 3-1/4 would be a better choice in every way except for the fact that you can't buy cartridges at your corner gun shop.

Lower pressures do make for easier extraction, and it has to be admitted the double does not have the extraction power that a good bolt action does when you have a sticky case. But this is not a question of how strong the action is or what it would take to blow it apart.





Edited by unspellable (04/08/06 07:16 AM)

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Marrakai
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Re: Double Rifle Pressure Recommendations [Re: unspellable]
      #60797 - 04/08/06 09:32 AM

Unspellable, you are most correct again!

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