Question about pressures

SnkBit

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I am wondering why the published load pressures for different powders vary by so much on the max load recommendations. Looking at this years Hodgdon loading data for a 168gr .308 bullet the max load for IMR 4320 is 46gr @ 59,300psi. The max load data for Varget is 46gr @ 50,600psi. Why do they not work up the Varget load to the higher pressure limits that they did the IMR powder? What causes them to stop 9,000psi short of where they stop with another powder? I see that the industry standard max pressure for the .308 is 52,000psi, why would they go beyond for some loads and not others? Just looking for a little insight on how the testing is done and why those choices are made.
 

Fuzzball

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Re: Question about pressures

"I am wondering why the published load pressures for different powders vary by so much on the max load recommendations."

Sorry, but 'they' haven't told us either.
 

Greg Langelius *

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Re: Question about pressures

True. Partly this is because many factors that are unique to each individual firearm will have significant (i.e. related to safety) influence on the actual pressures that it generates with a given load.

More importantly, how is the average handloader going to tell the difference between 55K and 65K?

When I begin a load development project, I start with pressure testing, using incremental charges and reading pressure signs from each fired case before proceeding on to the next larger increment. When I get to the pressure I feel is 'comfortable' I note it and break down any remaining cartridges. I treat that charge as the 'real max' for that gun and component combination.

'Pressure signs' and 'comfortable' are terms that relate to my own personal experiences, and have zero bearing on any other matters. I will not be making any further explanations of my meanings. Seek such answers elsewhere, from reliable sources.

How much pressure do you really need?

Pressure equates mostly to velocity (i.e. when pressures increase and velocities don't, you're well over max).

All the velocity one really needs is enough to stay above transonic at the target, wherever that is. I like 1300fps as a nice round number. if you're gettin' 'em out there significantly faster, do you really need to?

Laser trajectories can make wind adjustments easier, but they come at the price of more frequent barrel replacement.

Rather than flogging the very best friend you have on that line (your rifle), so's you can take it a bit easier on the wind doping isn't lazy. I won't use that word here. But I will be thinking hard about words like 'inconsiderate' and 'extravagant'.

Sure, the wind switches and you get caught out. We probably all did right along with ya. Your rifle will maybe drop one less point than mine.

To me, the wind is sorta like a 'Fool me once, shame on you. Fool me twice, shame on me' kinda thing. If the wind keeps foolin' me, I'm not gonna blame it on the gun. And I'm not gonna whip the poor thing into a smokin' frenzy trying to avoid making excuses for my own cruddy wind doping capacity.

Greg
 

Clark

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Re: Question about pressures

Management memo to load data publication staff:
<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">The optimal published load is that they should be half way between a stuck bullet and over pressure.

That would make the maximum safety margin in both directions.

The listed velocity should be the highest in the industry.

The loads for publication should be developed by spending no money. Just make them up, or reduce some other company's loads.</div></div>


 

SnkBit

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Re: Question about pressures

Clark thats some funny shit.

So what I'm thinking is that they call it quits when pressure begins to start rising sharply and velocity ceases to rise at a proportionate rate. Basically the point of diminishing returns.
 

Fred_C_Dobbs

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Re: Question about pressures

Probably something to do with burn rates and pressure curves. I jinned up these in QuickLoad using the first whole grain below SAAMI max:

36434424.jpg


I hadda shrink them to make them presentable so they've lost some detail so here's the numbers:

IMR 4320, #72 on powder burn rate chart
PMax: 59,598 psi
Distance down barrel: 1.2"
Velocity @ PMax: 934 fps
Time to PMax: 0.346 ms
Pressure @ Z1: 58,896 psi
Distance to Z1: 1.6"
Velocity @ Z1: 1084 fps
Time to Z1: 0.376 ms

Hodgdon Varget #105 on powder burn rate chart
PMax: 57,912 psi
Distance to PMax: 1.2"
Velocity @ PMax: 916 fps
Time to PMax: 0.352 ms
Pressure @Z1: 57,771 psi
Distance to Z1: 1.1"
Velocity @Z1: 834 fps
Time to Z1: 0.339 ms

QL, for whatever reason, sees no need to limit Varget to 50K psi.

I'm just guessing but a couple of possibilities come to mind.

One thing that sticks out about this comparison is that Varget's Z1 occurs considerably sooner than 4320's. Z1 is the point at which the production of gas ceases to accelerate (transitions from progressive burning to digressive burning). In fact, Varget experiences Z1 <span style="text-decoration: underline">before</span> it reaches PMax. 4032 experiences Z1 considerably <span style="text-decoration: underline">after</span> PMax.

I backed the Varget charge down to one that only had a PMax of 50K psi and the Z1 <span style="text-decoration: underline">still</span> occurred before PMax but the time difference narrowed from the 0.013 ms above to just 0.008 ms. The 50K Z1 also occurs after 1.2" of bullet movement (vs the 1.1" above), which makes 9% more bore volume available for combustion. That makes me wonder if Hodgdon knows something evil (for lack of a better term) happens when there's so little volume available and Z1 is so far in front of PMax. Like maybe the powder would detonate.

Another possibility is that is has something to do with bullet length and obturation. I didn't bother to measure one but according to QL, the 168-gr SMK is 1.215" long. Maybe Hodgdon was manipulating the Varget pressure curve to delay Z1 until the bullet has moved nearer its own length as a means of accounting for differences in seating depths and to ensure that full obturation has occurred before digressive burning begins.
 

500grains

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Re: Question about pressures

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Clark</div><div class="ubbcode-body">Management memo to load data publication staff:
<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">The optimal published load is that they should be half way between a stuck bullet and over pressure.

That would make the maximum safety margin in both directions.

The listed velocity should be the highest in the industry.

The loads for publication should be developed by spending no money. Just make them up, or reduce some other company's loads.</div></div>

I would agree with this assessment. In the past I actually liked some Layne Simpson articles because he had real data, not panty-waist data. Bob Hagel did too. For years I use a .338 Win Mag with a 250 grain NOsler partition over 76 grains of H4831 because Bob Hagel published the load. It was perfectly safe in my rifle. Now compare that to published loads today.


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