Judging Pressure

[turbo54]

Say you've got two different loads and you're trying to compare pressure... Both use same primer, brass, brass prep and are shot from the same rifle. Both loads use the same powder. Only difference is bullet and charge:

Load one leaves primers quite flat, and occasionally leaves a FAINT ejector mark...say once every 15 shots...

Load two doesn't flatten primers at all, but leaves a slightly worse ejector mark, and a bit more often...say once every five or six shots..

The loads in question are 308. 45.5 Varget/175smk and 46 Varget/178hpbt

Both are loaded on the lands.

Which is higher pressure?

To me, the brass flowing into the ejector hole is the last word on pressure, but if theres enough pressure to cause that, why aren't the primers flattening out?

 

[RotARy15]

Re: Judging Pressure

I'm still fairly new to reloading, but I don't care about primers.

I've seen primers get knocked flat and crater with 42.5gr of Varget under a 168gr Hornady BTHP. On the other hand, I have seen primers that looked perfect after firing a round that had stuck the bolt.

 

[The Mechanic]

Re: Judging Pressure

Difference in peak pressures and time at peak pressures. The flash hole is a set size or restriction. You can get more pressure at the primer through the hole with a higher pressure, or you could have a lower pressure for a longer period of time get through the hole and act upon the primer. The dent is more of a peak pressure indicator not a time indicator. Example would be a .454 casull pistol and a 45/70. Way different pressures and WAY different felt recoils.

 

[turbo54]

Re: Judging Pressure

I don't believe the flashole is reducing the pressure on the primer. A throttle controls flow, and therefore pressure in some cases, but there is essentially zero flow through the flashole. Thats my understanding of fluids and heat/mass transfer anyway.

No doubt peak pressure and "area under the curve" pressure will affect things though.

 

[KYpatriot]

Re: Judging Pressure

Turbo have you actually observed this? Seems to me for that to happen there would have to be inconsistency in either the brass or the primer.

 

[turbo54]

Re: Judging Pressure

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: KYpatriot</div><div class="ubbcode-body">Turbo have you actually observed this? Seems to me for that to happen there would have to be inconsistency in either the brass or the primer. </div></div>

Yep - noticed it today. Looked back over some development loads from two weeks ago - same thing. Seems strange.

 

[X-fan]

Re: Judging Pressure

Have you chronograph velocities?
Given the same propellant more speed means more pressure.

Primers can be very unreliable pressure indicators.
I have a strain gauge pressure tester and by the time most primers look flat the pressure is right off the scale of what my equipment can read....Or higher than 80,000psi.
I have also seen primers flatten at 60,000 (standard pressure) and one of my 348 Winchesters flattens them at 40,000 (probably action spring?).

FWIW I have seen brass ejector marks appear from 65,000 psi (Federal) to off the scale (Lapua).
Winchester, Remington and Nosler show up at around 70,000.

You can count on the brass being inconsistent in hardness/thickness or whatever it is that gives reliable ejector marks. Some show pressure and some do not.
That said if some show pressure you have pressure.

Tough call when your accuracy node is really on the pipe, but have you tried backing it down a couple of tenths?

 

[turbo54]

Re: Judging Pressure

The 45.5gr/175smk goes between 2655 and 2675 depending on weather. The 46gr/178hpbt goes 2745 in 45°(ish) weather.

I recognize primers are not a good indication of pressure, BUT, I am very accustomed to tge 45.5/175smk load, and the primer flatness is very consistent, time and time again. I've only fired 15-20 with the 178hpbt, but so far, the primers are also consistent - but show no flatness at all.

It's weird.

 

[Goin'Hot]

Re: Judging Pressure

Have you noticed any difficulty in bolt lift with either load?

 

[turbo54]

Re: Judging Pressure

Never. I consider my 45.5/175 load safe, but max. I consider the 46/178 load almost safe, but a tick hot...

I just don't understand why one max load flattens primers, and another max load doesn't.

 

[The Mechanic]

Re: Judging Pressure

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: turbo54</div><div class="ubbcode-body">I don't believe the flashole is reducing the pressure on the primer. A throttle controls flow, and therefore pressure in some cases, but there is essentially zero flow through the flashole. Thats my understanding of fluids and heat/mass transfer anyway.

No doubt peak pressure and "area under the curve" pressure will affect things though.</div></div>
Zero flow=no deformation of any kind on primer. Don't attempt to experiment with very large primer holes.

 

[X-fan]

Re: Judging Pressure

Your load looks pretty reasonable.

Got picture of both flattened and non flattened?

Do you ever measure your cartridge headspace with an instant indicator or whatever? Not to assume anything, but I have seen very light ejector marks from headspace.

Have you examined the case heads with a magnifying glass?
Look for faint ejector extrusion without the shiny spot.

 

[X-fan]

Re: Judging Pressure

You ever get this figured turbo?

 

[AtOne]

Re: Judging Pressure

Turbo I ran my set up through a program I can't name here

What it looks like is that the volume of the case occupied by the bullet is much more with the 178 hpbt, that coupled with the powder charge the difference in pressure between the 175's and 178's for me would be about 7,600 PSI pushing the 178's to the dangerous side of the program.

Even when I set the grain weight to 46 on the 175's its still 5,600 PSI lower than the 178's so I'm pretty sure its got to be the amount of bullet seated into the case.

Again this is for my load so take it as a reference but for me I would say 175's have less pressure.

Good luck

 

[MALLARD]

Re: Judging Pressure

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: AtOne</div><div class="ubbcode-body">Turbo I ran my set up through a program I can't name here

</div></div>
what program ? aww come on , i want to know !

to the OP, like you said , i'd have the feel ejector marks are a more clear sign of a high pressure load. So i would assume that load is hotter.but with that being said , sometimes brass is soft and shows marks even though the load isn't really hot. So is it a high pressure load , i can't say.

I like The Mechanic's idea of time to pmax being a factor. It sounds good, i will come back to this idea in a second.

I dont think you should look at primer flatness as an indicator of pressure. I have flattened allot of primers at low pressure. All sorts to, br2's , br-4s , fed mag match , even the 50 primers. So i stopped looking at flatness awhile ago. I have ( and i think you might have to) noticed variations in flatness from the same load, same everything and some primers are flatter then others. Assuming there is minimal (as much as possible) variation in CC, bullet weight , charge weight, flashole, primer pocket,ect,How does one explain difference in flatness coming from the same load?

back to the mechanic's point , i have a pressure trace machine so i have observed time to pmax , simply put , i get pretty consistent time to pmax and still get variations in flatness from the same load (this is what made me give up looking at flatness as an indicator, shit that is why i bought the machine in the first place, to help me rullout bullshit)

to take things a step further , i believe the primer blows back the during the intial .20 milliseconds ( probably sooner) of the event, time to pmax is usually .5-.7 m/s , so in theory , the primer has flattened before max pressure is reached... ??

but why is this happening ? ( variations in flatness from the same load)

my inept guess , differenecs in the primer pocket , some are "holding" on tighter to the primers then others. I have an LEwilson swage tool , i think it cuts off center , or that the bearing and shellholder are not lined up correctly , so there is some runout which leads to uneven cuts which supports my theory ( variations in primer pocket tension are effecting flatness... though that doesnt address the OP)... maybe i'll devise a way to measure that... ( other then my little force gage measurement thingy)

i dont think it's difference in primer cup thickness, its possible but since i can't control it... i have to rule it out.

i'd love to hear more thoughts.. maybe im on the wrong page... or reading the wrong book

 

[turbo54]

Re: Judging Pressure

Here are some pictures:


The two columns on the left are 45.5/175smk, the two columns on tge right are 46/178hpbt:

Again, the 4 on the left are 45.5/175, and the 4 on right are 46/178:

Left columns, 45.5/175, right 46/178:

 

[turbo54]

Re: Judging Pressure

Not sure how obvious it appears in the pics, but EVERY SINGLE 45.5/175 primer is flatter than EVERY SINGLE 46/178 primer.

Yet, of the cartridges pictured, NOT ONE of the 45.5/175 cases has an ejector mark, meanwhile several 46/178 cases do have a light ejector mark.

 

[vman]

Re: Judging Pressure

 

[YAOG]

Re: Judging Pressure

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: X-fan</div><div class="ubbcode-body">Have you chronograph velocities?
Given the same propellant more speed means more pressure.

Primers can be very unreliable pressure indicators.
I have a strain gauge pressure tester and by the time most primers look flat the pressure is right off the scale of what my equipment can read....Or higher than 80,000psi.
I have also seen primers flatten at 60,000 (standard pressure) and one of my 348 Winchesters flattens them at 40,000 (probably action spring?).

FWIW I have seen brass ejector marks appear from 65,000 psi (Federal) to off the scale (Lapua).
Winchester, Remington and Nosler show up at around 70,000.

You can count on the brass being inconsistent in hardness/thickness or whatever it is that gives reliable ejector marks. Some show pressure and some do not.
That said if some show pressure you have pressure.

Tough call when your accuracy node is really on the pipe, but have you tried backing it down a couple of tenths? </div></div>

X-fan,

Which strain gauge pressure system are you using? I've been considering purchasing an RSI Pressure Trace II system.

 

[KYpatriot]

Re: Judging Pressure

Turbo since you reloaded the 178s in a separate process than the 175s, it could be something related to the process itself. For instance did you use a different lot of brass? Was there any kind of lubricant residue on the brass of the 178 rounds that could prevent the brass from gripping the chamber leading to extra bolt thrust?

 

[turbo54]

Re: Judging Pressure

All brass comes from the same batch. All brass processed as one batch. A batch is 300pcs. When all 300 have been fired, they get decapped, tumbled, annealed, trimmed, resized, tumbled again, and expanded with a sinclair expander. There is certainly no residual lube - I tumble in stainless.

Both loads are "on the lands", such that I can feel the ogive "lead in", but yet not get so stuck they can't fall out of the chamber if you tip the muzzle up.

 

[X-fan]

Re: Judging Pressure

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: YAOG</div><div class="ubbcode-body">

X-fan,

Which strain gauge pressure system are you using? I've been considering purchasing an RSI Pressure Trace II system.


</div></div>

Yes, the RSI.
It works very well.
It uses a PC laptop as an oscilloscope so FYI you do need a laptop.

Up until recently I also owned an Oehler 43, but it was getting older and my buds discovered the CED (with infrared screens) was more a tiny bit more accurate (and a lot quicker to set up). I got offered the right money and away it went.

I'm thinking you will find the 178 probably has a longer bearing surface or is doing something to raise your pressures more than the 3 extra grains would indicate. Velocity and charge weight are up so pressure must fall in line. If you add in more potentially bullet drag/shot start pressure due to bearing surface or construction pressure could again be higher.

Things like this are what caused me to spend a small fortune on the 43.
A guy (or better yet a groups of guys) can be into a RSI comparatively cheap, but in the end it will confirm what you already suspect and what the cases/chronograph tells you.

There are a few strange surprises that make you happy to own a pressure test unit. Other experience will make you cringe at what others publicly suggest as safe.....Firearms steel is stronger than you would think!
If it wasn't there would be a lot more dead bubbas!

Me included.

 

[Greg Langelius *]

Re: Judging Pressure

IMHO, pressure is like an elephant. It's really hard to wrap your arms around the whole thing. It may even be dangerous to try.

I don't try to measure or compare pressures.

While working at higher pressures may appeal to some, it doesn't to me.

It's not that I'm afraid to; it's rather that I've been to that well many times, and over time I've found that the water there isn't as sweet as I had imagined. So little to be gained overall, and not without risk as well. Running high pressures comes with a price, it's not so immediately obvious, and it could be a serious, negative 'game changer'.

I look at pressure signs as something like venturing into the badlands and seeing skeletons strewn in my path. There's a lesson there for all to see, if only they will look and learn.

When I see mixed pressure indicators, ambiguous pressure indicators, or contradictory pressure indicators; I've learned to stop asking questions about which, and why, and how.

For me, the only real question is whether anticipated gains outweigh indistinct and insufficiently understood risks. Decades of experience have taught me that the answer is usually 'no; not really'.

So, too, shall you also learn. Of this I'm purty sure.

Greg

 

[X-fan]

Re: Judging Pressure

I know what you mean Greg,

Kind of reminds me of Cheech and Chong and the dog poop.
Looks like crap, feels like crap, smells like crap, tastes like crap...Good thing we didn't step in it!