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Old June 19, 2013, 10:35 AM   #1
stubbicatt
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Low pressure cartridges, how to determine dangerous pressures?

The cartridge I am considering is the 7.62x54r. As I understand it, max pressure for this cartridge is somewhere in the 45,000 PSI range. In comparison, maximum pressure for a 308 is closer to 60,000 PSI.

At 60,000 PSI or thereabouts, there are telltale signs of high pressures such as flattened primers, maybe primer craters, increased bolt lift force, ejector marks on the case head, and if measured, increased pressure ring expansion.

At 45,000 PSI, these indicia which are present at 60,000 PSI will be absent, assuming brass casings, commercial primers, etc.

How can one effectively determine that his load has not exceeded the maximum safe pressure for lower pressure cartridges?

I understand not to exceed maximum charge weight in published load data, but in some instances involving other cartridges, IME, I have determined that even maximum published load data is too high pressure, and stop my load workup before reaching that point.

Are there any methods any of you is aware of to determine when to stop in your load workup for 7.62x54r?

Thanks.
Regards,
Stubb.

Last edited by stubbicatt; June 19, 2013 at 10:38 AM. Reason: spelling
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Old June 19, 2013, 01:13 PM   #2
griz
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The same question arises with pistol rounds. Expecting a 20,000 PSI 45 ACP load to show the same pressure signs as a 60,000 PSI rifle load is wishful thinking. I just use the book data to find starting points and increase the charge until I reach factory velocity. Before I got a chrono I went strictly by the book, and except for the rare times I needed max power, I was happy with middle of the road loads.
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Old June 19, 2013, 01:24 PM   #3
mehavey
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The 7.62x54R is not "low pressure" by modern/reloading standards. The confusion
comes from the use of different pressure units: CUP vs PSI.

I Kwik-checked QuickLoad to find a standard of 56,565psi. Other sources
will reference 46,000CUP. The right answer is... both are correct.

Take a look here for a "general" CUP-to-PSI discussion
http://www.shootingsoftware.com/ftp/psicuparticle2.pdf

and here for the actual Russian cartridge:
http://kwk.us/pressures.html

...so don't lose any sleep over it.

That said, resize/headspace for the shoulder when reloading -- as you should for
most all rimed and/or belted cases. Otherwise you risk head separation from repeated
case stretch after a bit.**



**
A 'bit' is defined as "a few"

Last edited by mehavey; June 19, 2013 at 01:29 PM.
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Old June 19, 2013, 01:28 PM   #4
Unclenick
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Mehavey,

You've got two identical links there. Did you maybe mean for the second link to be this one, which uses the 7.62×54R as the example under test?


Stubbicatt,

As that article suggests indirectly, you load to the reaction of the gun steel and the brass to determine a limit for your particular gun and brass. If either one shows regular signs of being overstressed, you need to back off. You just can't depend on uncalibrated brass to give you a pressure estimate, and can't count on any single piece of brass to behave in a way that represents the whole lot, as the article also demonstrates. Overstressed, in this case would have to include semi-auto actions not being battered heavily. There are some instances like that in which experience is the best teacher. I keep a generalized list of pressure signs here.
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Last edited by Unclenick; June 19, 2013 at 01:37 PM.
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Old June 19, 2013, 01:34 PM   #5
mehavey
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Actually I meant this one:

http://kwk.us/pressures.html
7.62x54R
PSI: 57,000
CUP: 46,000

...which lays out case-by-case (no pun intended) comparison for each cartridge.

I've corrected the link in the original post, but your own link brings out another pressure issue -- head expansion. Bramwell's article should be required reading for what CHE says -- and doesn't say.

Last edited by mehavey; June 19, 2013 at 01:40 PM.
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Old June 19, 2013, 02:20 PM   #6
Strafer Gott
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Roger that for pressures with the 7.62x54. I've always considered the big Russian to be more than a match for the military30-06, and definitely the .308.
Most of the NATO .308 is loaded a little light, to preserve op rods and what-not.
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Old June 19, 2013, 04:40 PM   #7
jmr40
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Quote:
Are there any methods any of you is aware of to determine when to stop in your load workup for 7.62x54r?
I don't load for the round, but I'd do it the same exact way I'd do it with any other round. I'd use a chronograph. Trying to use

Quote:
flattened primers, maybe primer craters, increased bolt lift force, ejector marks on the case head, and if measured, increased pressure ring expansion.
as a guide is not a good idea. These things don't happen until you are well over 60,000 psi, closer to 70,000.


If your loading manual claims 50.5 gr of Varget is good for 2985 fps with 150 gr bullets @ 46,400 CUP is a max load, ( I borrowed this load from here). http://data.hodgdon.com/cartridge_load.asp then 2985 fps is a max safe load. By starting low, and working up you know you are approaching a safe max load when you get close to the max speed. You might hit 2985 fps at only 49.5 gr. Going with more powder will put you over pressure.

A chronograph isn't perfect, but is the easiest way to predict pressure BEFORE you get over the limit. Other methods require you to go over the limits, then back off some. You could still be over pressure and have no traditional signs.
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Old June 19, 2013, 07:51 PM   #8
stubbicatt
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JIMR40, I have always used pressure ring measurements as a gage to ascertain when I am approaching max pressures, by comparing factory ammo measurements to new brass measurements, with my loads fired in it. This is my "Do not exceed" measurement.

Do I understand you to suggest that a chronograph is a measure of max pressure? Aside from the differences in barrel length from the firearm in which the publisher obtained its data, there are many vagaries in chamber dimension, bore diameter, etc., which would make me reluctant to use a velocity as a measure of maximum safe pressure.

Mehavey, without resorting to custom sizing dies, how do I resize for the shoulder on this rimmed case? I mean, if the shoulder were only blown forward say .004" as one might find on a loosely chambered, modern sporting rifle, chambering a rimless cartridge, I can follow you just fine. Here, in rifles with very long chambers, which headspace on the rim, which often exhibit shoulders blown .1 to .2 inches forward, there is no way I am aware of to use a conventional FL die to bump the shoulder without squeezing everything out the side, where the die cannot touch the case. This will prevent the cartridge from chambering again. --Please share with me what you have learned.

Thanks all for your input and your learning.
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Old June 19, 2013, 09:47 PM   #9
mehavey
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Partial resize: same thing I do for my 300Win, 375H&H, and 303Brit... all of which are bottle-neck cases that are either rimmed, belted, or semi-rimmed. And if you don't already have it, get Hornady's Headspace Comparator set for these steps:

1. Run a headspace/comparator measurement on a fired case (or several) that still fit
back in the chamber and the bolt close. That will be your baseline shoulder dimension.

2. Screw the sizer down to the shellholder, then back off a quarter turn.

3. Size a case and see if the shoulder dimension has lengthened. If it has, see if the
case still will chamber w/o resistance. If it will, that is your new shoulder dimension.

4. If it won't chamber, screw in the die a 1/32 - 1/16 turn and size again until the case will
chamber/bolt close. Measure the shoulder and that's your new headspace standard.

From this point on you merely have to screw the sizing die in far enough to produce this
shoulder/headspace dimension to prevent excessive stretch.

Last edited by mehavey; June 19, 2013 at 09:54 PM.
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Old June 20, 2013, 09:19 PM   #10
Clark
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CAUTION: The following post includes loading data beyond or not covered by currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assume any liability for any damage or injury resulting from use of this information.

~2003 I put 180 gr Sierra .311" soft point bullet and 40 gr bulk IMR4895 [that batch behaves like canister H322] in a Mosin Nagant and worked up until I got a sticky bolt at 50 gr.
That is starting at ~ 40 kpsi and working up to ~ 74 kpsi

2009
Same bullet same powder, but Lapua brass, not S&B
47 gr 27.5" barrel chronographs at 2795 fps
Quickload predicts 61 kpsi 2820 fps, so it is working well.
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Old June 21, 2013, 01:53 PM   #11
mehavey
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2,800fps w/ a 180gr bullet is nothing to sneeze at even for a grown-up's 30-06 -- much less a 7.62x54R.
(But then again, that sneaky Russian's got 3½ extra inches of barrel.
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