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Too much pressure in the 300 RUM?

 
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  #29  
Old 04-03-2005, 05:25 PM
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Re: Too much pressure in the 300 RUM?

BB wrote:

And the only ez way to calibrate it is to fire over-pressured loads that show signs of overpressure - then you can use it to get a safe max.

Actually, neither PT or the M43 require calibration. Without a SAAMI spec pressure barrel chamber, even SAAMI pressure testing reference ammo is not going to calibrate one. The only way I know of that will truely calibrate your chamber/action/strain gage combination is to hydraulicly pressurize it to peak operating pressure. A specific offset pressure could "then" be determined. Firing over-pressured loads will only tell you that they are over-pressure, not by how much though. If you use over-pressured loads in order to calibrate a strain gage system, the SG system is only as accurate as the method used to determine the over-pressured load, and if you suggest using classic excessive pressure signs to determine this, you now do not even need the SG system. Truth is, the SG system is far more accurate than most can believe... with no calibration at all. The thick and thin wall pressure vessel formulas used in the M43 and PT software are public knowledge. The modulus of barrel steel and brass are well known and each strain gage is calibrated before you get it with a known gage factor. The rest is pretty simple. I would guess that PT is accurate to within 3 Kpsi, and more than likely within 1-2 Kpsi. Test box after box after box of factory ammo for multiple cartridges and you may decide it is just coincidence but, when ammo tested comes within this 0-3 Kpsi range of SAAMI rated pressure spec's for a given cartridge every time... you realy have to question how much ammo people have truely tested using SG systems when they talk about the inaccuracy or need for calibrating these SG systems mentioned. SAAMI reference ammo fired in yours or mine's chamber should be at the same, or lower pressure than what it produces in a SAAMI test barrel's tighter chamber, right? Yes, that's the purpose but, it does not mean that if your SG system registers 60 Kpsi and not the expected SAAMI 65 Kpsi this cartridge is rated for, and you expected to see, that your SG system is not calibrated correctly.

I have never seen factory ammo exceed SAAMI maximum average in any test that I've run, nor have I seen the average pressure between rifles chambered for the same cartridge vary in pressure more than about 3 Kpsi. Needless to say, this is pretty damn close to what I would expect, and I'm quite confident in developing loads from the ground up for a wildcat cartridge with the aid of the SG systems I use.


Getting MV by using an "accurate" chrono will probably get you closer to actual max load than CHE will in most cases, and it is a very good indicator of pressure contrary to the nay-sayers. However, it may allow you to go a bit beyond max load if using it as a sole indicator because it is not as conservative as CHE. Used in conjunction with other excessive presure indicators it can very often be the most helpful.
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  #30  
Old 04-09-2005, 07:33 PM
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Re: Too much pressure in the 300 RUM?

Brent Moffitt writes:
[ QUOTE ]
Firing over-pressured loads will only tell you that they are over-pressure, not by how much though.

[/ QUOTE ]
EXCELLENT Point - I was assuming I could just nudge it over the top - but I think you are right.

[ QUOTE ]
neither PT or the M43 require calibration. ... The thick and thin wall pressure vessel formulas used in the M43 and PT software are public knowledge. The modulus of barrel steel and brass are well known and each strain gage is calibrated before you get it with a known gage factor. The rest is pretty simple.

[/ QUOTE ]
Except you don't know the exact geometry of your receiver, exact modulus of barrel batch (altho I suppose that could be very accurately measured).

I know folks have calibrated their SG with a hydraulic press. It would be interesting to know how close they are. I have no evidence how far off they are, I'm just skeptical (being an applied mathematician where almost every measurement needs to be calibrated).

I totally agree that SAAMI loads are same or lower (I think lower) but that is a much simpler task - loads get erratic the closer you get to max pressure.
[ QUOTE ]
I have never seen factory ammo exceed SAAMI maximum average in any test that I've run

[/ QUOTE ]
agreed
[ QUOTE ]
nor have I seen the average pressure between rifles chambered for the same cartridge vary in pressure more than about 3 Kpsi.

[/ QUOTE ]
Now that's an excellent test I have never considered. Have you SG tested the same batch of ammo in different guns? I guess on conservative loads it's not too surprising, but when you bump up near the max the differences could get significant.

[ QUOTE ]
Getting MV by using an "accurate" chrono will probably get you closer to actual max load than CHE will in most cases

[/ QUOTE ]
How so? All that will do is get you to the conservative max which is probably significantly below SAMMI max pressure. Would you say that's true given my excellent .0001 blade mic, marked brass and experienced measure-er (you not me [img]/ubbthreads/images/graemlins/smile.gif[/img])

Now you've got me spooked out of using CHE, measured bolt release force, primer inspection, etc and back to my old scout masters maxim (NEVER EXCEED PUBLISHED MAX LOADS).

Question:
Under safe loads the CHE during firing conforms to the chamber THEN springs back to less than .0001" of original (once fired) dimensions after pressure is released. When you exceed the elastic limits of the cartridge brass, it doesn't spring back. Is that how CHE measurement works?

Awesome info Brent - the mathematical skeptic in me still wonders - your probably right about this stuff.

Discovering Maximum Load By Measuring Case Head Expansion
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  #31  
Old 04-10-2005, 04:06 PM
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Re: Too much pressure in the 300 RUM?

BB,
Just saw your post here but I've got to run out the door, back this evening if I get the chance.

Pays to be skeptical, people make mistakes, draw wrong conclusions and the list goes on and on and on. I wouldn't go so far as to say I'm right about anything but rather it just "might" be something else to consider.
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  #32  
Old 04-12-2005, 06:50 PM
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Re: Too much pressure in the 300 RUM?

[ QUOTE ]
Except you don't know the exact geometry of your receiver, exact modulus of barrel batch (altho I suppose that could be very accurately measured).


[/ QUOTE ]

How many PSI do you think the estimated PSI numbers are off considering there may be some error with reguard to the barrel steel modulus? Exact geometry of the receiver, you'll have to explain what you mean a little more there, I'm not sure I follow.

[ QUOTE ]
I know folks have calibrated their SG with a hydraulic press. It would be interesting to know how close they are. I have no evidence how far off they are, I'm just skeptical (being an applied mathematician where almost every measurement needs to be calibrated).


[/ QUOTE ]

I've never heard of anyone that has done this, myself, but like you, I'd be interested in their findings as I'm sure someone has done it. If I was ever concerned with the numbers I routinely see I would have went there to test it already. I can tell you that getting numbers from the receiver mounted SG is not workable and have learned quite a few things testing it before putting a barrel block on my 30/338 Lapua Imp. Pressure was 20-30 Kpsi lower than the barrel mounted gage though, and although it seemed to move up and down with pressure changes it was not consistant nor linier, sometimes just a bit erratic. A 5 Kpsi actual change sometimes registered 1 Kpsi on the receiver gage, or 10 Kpsi change indicated just 1-3 Kpsi etc., sometimes it was 5 kpsi..... it did not instill confidence in any way, still it was interesting. Burt at Southwest Products has many or all of those test files too.

[ QUOTE ]
Now that's an excellent test I have never considered. Have you SG tested the same batch of ammo in different guns? I guess on conservative loads it's not too surprising, but when you bump up near the max the differences could get significant.

[/ QUOTE ]

Yes. That's what I was trying to say, and significant it could get, could. I've just not seen it, even though significant is a little subjective. The closest thing I've seen, and wish I'd been set up to test both rifles was, two 300 WM's that shot the same ammo about 80 fps apart, same length bbl and all. The Ruger was faster than my Rem with 200gr SGK's over the Oehler 35P back to back. All of my Rugers have not been slow shooters so it was not too suprising but, still a few things interest me about the WHY. When you get the SG system I'm sure you will feel there is some error but shortly find it must be rather small indeed, way smaller than I would have ever anticipated.

[ QUOTE ]
I totally agree that SAAMI loads are same or lower (I think lower) but that is a much simpler task - loads get erratic the closer you get to max pressure.


[/ QUOTE ]

I've seen this in some but, most were still predictable at the top end. I think most of what I've seen in the past was due to bbl temp increasing and some loads that run on the high end with certain powder did not stay consistant for a string of shots over three. I've just seen it but, don't know why. I've seen a 6.5 WSM load that started out at 65-67 Kpsi or so keep climbing in PSI and MV for ten shots until it damn near locked the bolt as it approached 75+ Kpsi, when the 63Kpsi load would stay stable for twenty shots if you ran that many, all with ES and SD VERY tight. Friggin amazing some things. The more you test the more you learn there is just more and more to friggin test!

[ QUOTE ]
How so? All that will do is get you to the conservative max which is probably significantly below SAMMI max pressure. Would you say that's true given my excellent .0001 blade mic, marked brass and experienced measure-er (you not me )


[/ QUOTE ]

I will take an Oehler and what I know of visual and felt pressure signs and use any day over that of CHE or PRE measurments with the best blade mic and techniques and would bet $1000 I'm within 2-3 Kpsi of 65 Kpsi, or 70 Kpsi.... you name which. CHE and /or PRE will simply not tell me this but "can" be helpful.

[ QUOTE ]
Now you've got me spooked out of using CHE, measured bolt release force, primer inspection, etc and back to my old scout masters maxim (NEVER EXCEED PUBLISHED MAX LOADS).


[/ QUOTE ]

You've probably got a pretty good handle on it, it sounds. At least half of my testing has been well beyond the redline, some really far beyond but, the number of rounds I've fired in the top end ranges has been so many, AND, having pressure and velocity numbers "instantly" available for the majority of them it's an association learning type thing that just developed. I feel the way the bolt lifts, the way the case extracts, I know the case's current condition, headspace etc, inspect the base and primer condition then it gets associated with MV and PSI numbers, somehow it sticks with you and you just know what pressure it's running without even looking after doing it so long.

Most handloaders do the very same thing but just relate it to some sense of "this is about right here" where pressure signs aren't excessive, and everyone's different, I just have a number in my head it's associated with, so there's not much of a difference except thatsome people "think" their loads are 65 Kpsi when they really run 70 Kpsi and some think they run 65 Kpsi when they really run closer to 60 Kpsi. It varies from person to person because of what they feel safe with and assuming etc, etc. I'm quite sure some don't want to know what pressure they run because they know they are safe, and like it that way, and some know they are running hot, and like it that way too...

[ QUOTE ]
Question:
Under safe loads the CHE during firing conforms to the chamber THEN springs back to less than .0001" of original (once fired) dimensions after pressure is released. When you exceed the elastic limits of the cartridge brass, it doesn't spring back. Is that how CHE measurement works?

[/ QUOTE ]

I don't think the CH ever expands to contact the chamber as it's usually along the lines of .005 smaller in diameter, I don't think it completely yields either or you'd probably never get the case out of the chamber as the barrel and receiver also expand but do not yield so they spring back. The casehead does show evidence it is in stages of yielding if it gets larger in OD. The limited duration at peak pressure probably keeps it from expanding completely and sticking in the chamber. The powder burn rate and bullet weight probably alter these effect on the brass as well.

There is no linier conversion from the copper crusher method to PSI as well. The CHE and PRE measurments offer too many variables but can be helpful, just not very accurate IMO.
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  #33  
Old 04-13-2005, 12:32 PM
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Re: Too much pressure in the 300 RUM?

Wow, thanks for all the info guys!! You sure do know your stuff!

Yeah, the load that I have is the MAX load published in the new Seirra Manual. I try not to load my rifles right to the wall, but sometimes that is just where it shoots best [img]/ubbthreads/images/graemlins/smirk.gif[/img] I think that I will cut the load back, I will be sacrificing about 1/8" to 1/4" in accuracy at 100 yards, but that is ok if it saves on pressure.

Thanks again guys!!
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