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Too much pressure in the 300 RUM?

 
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  #22  
Old 04-01-2005, 03:24 PM
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Re: Too much pressure in the 300 RUM?

[ QUOTE ]
Southwest Products PressureTrace is a very inexpensive method to attain the real pressure numbers, no guessing game

[/ QUOTE ]

Actually it's totally guessing until you calibrate it. 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.

Lou Boyd showed how to calibrate by using known pressure in the chamber - posted on ExtremeInAccuracy.com, but that site has been down for several days. When they fix the site I'll post Lou's data.

The strain gage you attach to your gun is using many assumptions.

I'm going to get a USB PressureTrace and I'll calibrate it with all the methods outlined in this thread.
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  #23  
Old 04-01-2005, 08:29 PM
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Re: Too much pressure in the 300 RUM?

[ QUOTE ]
The other 'best' way to monitor pressure is with a chrony.
Jerry

[/ QUOTE ]

Unfortunately the chrony is not very useful in establishing max safe pressure. Barrels differ too much to derive max pressure from other barrel data. You can establish a conservative max this way, but you have no idea how close you really are to the true max safe pressure. Even using a chrony you have to build up slow. A faster twist barrel may overpressure at safe velocities of a standard twist barrel.
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  #24  
Old 04-02-2005, 09:33 PM
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Re: Too much pressure in the 300 RUM?

I have a cheap RCBS dial caliper that I use at the bench and can pretty easily split a thou into at least 1/4 thou's. In other words, resolving .00025" increments isn't too difficult if I use consistant pressure and keep the blades clean. For taking relative measurments, this is completely repeatable with any caliper I've used and works just fine.

If you measured a once fired case for a baseline, then found that a twice fired case measured .0008 - .001" over that diameter, I'd say you are on the very upper end of a max load, possibly experiencing "some" bolt lift and/or extraction resistance. Cratering of primer, enough that you can begin to really catch a fingernail on, with the Federal primers usually begin to manifest at around 67,000 psi or more, sometimes not until 69-70 Kpsi. Over 70 Kpsi cratered rim is usually getting pretty high, .010 - .020 tall or so... definitelly very pronounced in the majority of fired cases.

In my experience, if expansion gets around .0005 - .0007 over, you are probably still safe but getting into higher than SAAMI rated pressure specs.

I would also venture to say that most people handloading for top MV and running at the red line would probably see somewhere around .0008 - .0012 increases in casehead diameter if they actually measured them... in a correct manor. I would also say that most of them run loads up into the 70+ Kpsi range without knowing they are, or some just really not caring. The rifle actions and brass will tolerate it and thus they are pushed to higher levels than the majority of handloaders load to.... because no ill effects are realized this is done.

If you want to go faster, the simple answer is to use a case that holds more powder. Other things can be done so you don't have to run tip top pressure to produce the very best accuracy the barrel is capable of, and it's my opinion that something is wrong if the rifle will not shoot top groups with anything other than a top load. It's either the bullet, powder etc, the rifle or the barrel but something isn't quite right. Best to do everything you can to accurize the rifle, use a quality barrel, the best mounts and scope you can buy to set yourself up for the very best possibility your rig will not be finicky and lead you on a wild goose chase in order to resolve a problem.
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  #25  
Old 04-03-2005, 03:03 AM
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Re: Too much pressure in the 300 RUM?

[ QUOTE ]
I have a cheap RCBS dial caliper that I use at the bench and can pretty easily split a thou into at least 1/4 thou's. In other words, resolving .00025" increments isn't too difficult if I use [sic]consistant pressure and keep the blades clean.

[/ QUOTE ]

Sorry, but that's impossible. How repeatable are your measurements? Are they good when you use consistent pressure? - then you are probably using pressure to get a consistent measurement.

Have you ever been tested with 9 pieces of brass of different sizes and done a blinded/randomized measurement to verify that you can get .00025" with a <font color="red"> cheap RCBS dial caliper? </font>

I've got several .0001 Mitutoyo and B&amp;S <font color="blue"> NOT CHEAP </font> micrometers - but these are designed to measure to .0001 <font color="red"> with NO interpolation </font> - there is no pressure guessing with the ratchet. I can easily interpolate 1/4 of .0001 reading on my mikes, but that doesn't mean I can rely on that reading.

Don't take my word for it, Stan Watson is an expert on the topic and he writes <font color="purple"> [ QUOTE ]
Measuring web expansion also requires a rather specialized micrometer, although belt expansion can be measured with a standard micrometer as long as it has a precision to the ten-thousandths (0.0001) of an inch. To measure web expansion, you will need a blade micrometer (left) with a precision of 0.0001 inch, and these are moderately expensive as well as being moderately difficult to find. Measuring such small increments also requires some skill, so if you are not already fairly skilled at taking precise measurements you will need to practice. The bottom line, though, is that in my judgment any measurable web expansion of a properly fire-formed case -- as happens when you fire a factory round in your rifle -- is an indication of excessive pressure.

[/ QUOTE ] </font>

I make these remarks not to be critical, but to tell others not to depend on interpolation with a cheap caliper when you should be using a good micrometer.

Stan on Pressure Indicators

Stan on pressure, some repeated info
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  #26  
Old 04-03-2005, 01:21 PM
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Re: Too much pressure in the 300 RUM?

BB,

Impossible if it is not done right. I don't moderate pressure to get the desired reading if that is what you suggest here? If a case expands .00025, you would have to use more pressure to get it to read .00025 less than this, same if it expanded .0005 or .001. More pressure flexes the blade to rail joint on the caliper and you can certainly be consistant and feel a difference in pressure used so this does not happen... with some practice. Is it the best way? No. A .0001 mic certainly is best. What I am trying to say though, is that if the case does expand, the needle will reflect it, with or without a line on the dial at each .00025 or .0005 increment. Yeah, you can use more pressure and see the needle move enough to make you second guess the small change you see but, you use consistant pressure and you can't deny that the case just expanded by the amount you see on the caliper. Could it be off a ten thou, or even two ten thou? Yep, but very doubtfully three ten thou if doing it with some care and consistant pressure. What does .0002 - .0003 CH expansion mean in terms of pressure if using a .0001 capable mic anyway? IMO it means very little because it is just another indicator that you are up in pressure but NOT that you know where pressure is AT. It is far too inaccurate and dependant on many other variables to ever be corralated to an actual pressure to use as a sole PSI indicator, so .0002 - .0003 in the realm of things probably means ~ 5-10 Kpsi. Is measuring CHE accurate enough in predicting pressure that ~ .0002 - .0003, or ~ 5-10 Kpsi is even very meaningful? It probably increases the overall error by around ~ 25% - 30% maybe. Are a lot of these figures I state here assumptions? Yep. Measuring CHE to the very best of ones ability with a top grade mic is also a BIG assumption when you try and relate it to actual chamber pressure because there are so many variables. The strain gage systems attemp to minimize these variables, and do so very well.

Anyone is foolish if they DO NOT use MULTIPLE indicators of pressure and instead rely on one indicator alone, and this includes anyone using a strain gage system too.

These are just my opinions based on my experences and anyone reading them should not read them as "definitive", but rather my simple observations with several assumptions thrown in. Something to think about and test for yourself if you so desire.


Stan says:
The bottom line, though, is that in my judgment any measurable web expansion of a properly fire-formed case -- as happens when you fire a factory round in your rifle -- is an indication of excessive pressure.


Stan has been testing with the 43 for years now, but his statement there is really incorrect, although it is conservative and may keep someone from venturing into the red zone, so it's understandable why he says this, still it is incorrect. ANY case will measurably expand across the web (CH) on firing #2 at 63-65 Kpsi, take it to the bank. Most will expand about .0005", even if the chamber's base is cut to less than a .0005 total clearance. The reason for it is that the barrel and receiver ring DO expand when peak pressure is reached, that is a simple fact, and the case will not hold its shape at those pressures... even though they are safe loads.

Don't take my word for it, or Stan's, expert or no expert, think about it for yourself, and test it yourself. I take very few people's word on very few things but, I do consider most everything I hear or read, but that's just me.


BB wrote:
Have you ever been tested with 9 pieces of brass of different sizes and done a blinded/randomized measurement to verify that you can get .00025" with a cheap RCBS dial caliper?

No I have not.
If you'd like, I could number 10 cases, wrote on tape, measure them, send you the only recorded measurments along with the cases so you could re-number them and return them to be re-measured. I'd send you the second set of numbers I recorded along with each case number. You match up the case numbers and you could tell us how good/bad I did? A third party could be in the loop to further validate the findings.

BB wrote:
I make these remarks not to be critical, but to tell others not to depend on interpolation with a cheap caliper when you should be using a good micrometer.

I totally understand, and agree it is best to use a good .0001 capable mic. As you've stated, it's pretty easy to see how repeatable you can be with a cheap caliper by measuring multiple cases. Record them, have someone re-number them and check them again and compare.

Had to run out and didn't get a chance to reply to your previous posts yet. Off again for a bit but back after a while.
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  #27  
Old 04-03-2005, 03:34 PM
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Re: Too much pressure in the 300 RUM?

I tend to think Stan is a bit too conservative too. You probably can get reasonable measurements with your caliper but I don't think most folks should, not without testing their measurements with a good mic.

I've got several fish scales, perhaps I could measure the force required to open my bolt.

Soft brass might be another good way - but it looks like case head expansion is still the best low cost method of finding max or slightly over max.
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  #28  
Old 04-03-2005, 05:04 PM
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Re: Too much pressure in the 300 RUM?

Snap mic anyone? OK here goes. Two guys standing side by side measuring the same object (in turn) using the same measuring equipment will not come up with the same reading! Does that make this method invalid? No. I have had this experience Measuring Berger bullet with Walt Berger measuring his bullets (the same bullet) I've done the same with Neal Jones measuring his product. Does this mean I or they are wrong? What all this means as to comparing between two people the use of the snap mic is the tool to use to eliminate reading errors between two people. For the individual taking readings it's different. He can monitor himself and that is why this works as a good INDICATOR of pressure. Strain gauges sure have there place, The limits on the strain gauge is unless you have a ctg that is longer than the receiver ring you're stuck, this is why the TC pistol is popular with guys for this kind of testing. Don't want to mess up that stock, epoxy that expansion ring? Then you can shop some where else. I'm not telling anyone that one method is any better than the other, I'm saying you got to understand the limits to each. [img]/ubbthreads/images/graemlins/grin.gif[/img]
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