All fired from my Rem 700 SPS Tactical 20"bbl 1:12 twist with a long freebore. Actual charge weights are not really relevant and many of them are over max so I don't really want to add those in the details. This is not for load data. Different powders may behave differently, this is all done with W748.

The rise times in the statistics section are an indication of powder burn rate in microseconds from 25% of max to 75% of max, a higher number is a slower burn rate.

For tldr:
1) In all of my testing ON MY GUN making the OAL longer decreased chamber pressure.YMMV
2) I have not been able to get a pressure increase when seated on or close to the lands.
3)With my LC11 brass, bigger swipes don't occur until mid to upper 60k psi, hard extraction starts around 69-70k psi.
4) Noticeable Fed210 primer flattening doesn't show until about 65-66k psi.
These pressure numbers are not exact, I have used a few methods to confirm the correction is close but there is really no way to know the exact PSI. Like a chronograph for comparison purposes you can trust it, and as with a chronograph the higher the number of shots will give you a better idea of what you are really getting. Many of my tests have been one shot of each. You have to look at the overall trend from all of the data and not focus on one specific reading. The extreme spread of PSI for identical 10 shot groups can be over 3k PSI so unless they are larger groups you should not focus on small changes from one shot to the next, instead look at one extreme to the other to see what is happening.





This was done in my bullet trap in my house, so everything stayed an even temperature. I got the muzzle velocities from from my MagnetoSpeed, shot number 4 was lost because the cable came disconnected and I couldn't see it in the trap. The chamber pressures are from my PressureTrace.
The load is 44.5gr of W748 in LC11 9x fired brass(55.55grn volume), 210 primers and 175gr SMKs out of my 20" Remington 700. The max  COAL I could run was 3.035" with the 175gr SMK, any longer and it would fall out of the case. This made for about .035"bearing surface in the neck. I didn't want to waste my $0.50 each 210gr Bergers on this but they do reach to the lands, when I get more of them I may try again but over a smaller distance from the lands as the results here are pretty obvious.

You can see that as I lengthen the COAL the pressure drops until I get to within .030" from the lands. This is due to the fact that I am increasing case volume and to a lesser effect I would think, decreasing neck tension a bit as well. When I get really close to the lands the pressure will start to rise again because, being close to the lands, the bullet has less momentum(less of a run at it) to push itself into the tight fit of rifling. In further tests I could not get a pressure increase in my gun for reason unknown.
175gr SMK


As requested by AeroE:
The percent is load density and the grain number is room available for this powder calculated by QL


Highest and lowest pressure cases. Head diameters were .4698" for low and .4706" for the high.











Update with 210gr Bergers seated to lands

I am going to do more tests with this, to see if I will repeatedly get no pressure increase on the lands. This not what I expected.
Here is a sketch with the extremes, trying to put every length in is too busy because they are so close together. I just lengthened the bullet to 210gr Berger length from the first because I am lazy and have no idea what the ogive radius is for the Berger.
The percent is load density and the grain number is room available for this powder calculated by QL



Highest and lowest pressure cases. Head diameters were .4699" for low and .4702" for the high.




--------------------------------UPDATE---------------------------
Below done at range, for all shots chamber temp was kept between 105-112 degrees. Ammo temp was between 89-91 degrees.


Another bunch of Berger 210 loaded to lands and back .200
3 shots of each this time for averaging. 1st shot was missed. I quit at .200 away from lands because pressure was already too high to go farther away from lands. Unsure why I see little or no increase in lands, I am actually jamming a couple thou. I can only do 10 shot strings so I had to make two charts.


The next four sets are together shot in the same session
210gr Load Workup at 4 different OALs same charge, 10 shots each
I did four 5 shot groups of 4 different lengths at the same charge and then repeated the four 5 shot groups for a good average there are 2 5 shot charts for each load length starting closest to lands. Now being 10 shot group averages, smaller changes may be more reliable indications of pressure change.

.085 off lands


.095 off lands


.105 off lands


.115 off lands


Higher and lower pressure cases from this bunch for comparison.














-----------------------------------------------------------------------------------------------------

For reference of what normal loads would look like

This is my normal 175gr load that I seat to max length to fit in my mags and I use a different powder charge. I altered the test rounds charge to account for the short seating depth. This particular was just a warmup load with shit, no prep, runout brass but for some reason made a nice 1moa 10 shot group the last 7of the 10 shots made .5moa  

The crosses are where the bullet exits the barrel by the way.
175gr smk 45.0gr w748 2.840"

175gr smk 45.0gr w748 2.840" (another day)


Hornady 168gr HPBT Match factory ammo fired right after.

Very informative !  Thanks for posting !

This is relevant to my interests. I had a very interesting chat not long ago with a retired Los Alamos weapons designer. He confirmed a few of my thoughts about what is ACTUALLY happening during the events you describe.

Thanks for taking the time to post this.

Yep, misinformation and part truths are continually parroted, no one bothers to question anything. Rifle ammo will see a pressure increase from deeper seating for the same reason handgun ammo does. It is commonly stated that longer COAL in rifle ammo will increase pressure leaving a person to think that shortening wont do the same. In this case I could go a hell of a lot longer and only see lower prerssure while only going a fraction of that distance shorter made for a dangerous condition.

I added a couple more charts with normal loads in the OP.

Great job. I'm glad you took the time to use your equipment to do this testing because the other thread just didn't intuitively sit right.  Also, I can easily see a sharp pressure rise would occur if the bullet is very close to the lands and groves as it's being swaged into them without the benefit of any momentum to aide it.  Not only did you settle the discussion with excellent test data, your explanation of the results 'makes sense' and is consistent with what one sees with pistol ammo OALs too.

Every six months or so, we get someone in here who argues that longer coal = higher pressure without exception. Real world experience indicates otherwise (see examples of everyone using pri, hk and other mags to get 2.3 coal in the ar so they can run faster), and I'm glad you did some real testing to prove that. If you could show how getting right on and jamming the lands does increase the pressure, that would be good, but this pretty much tells people what they need to know.

For the purpose of this test, I don't think it matters if his equipment is calibrated - only that it is consistent.

I've stopped posting in other threads on this topic because inevitably some "expert" throws up 3 charts "proving" that, in rifle, longer seating = more pressure. The worst part is the very charts the expert posts disprove his premise, and any comments pointing that out are ignored. In many chambers, for many starting COLs, longer seating = lower pressure until the bullet is close enough to the lands to increase pressure. Two factors working somewhat in opposition.

Added to OP, I think this is what you mean


I wanted to show a larger range to get a better idea and also to show short seating is also bad.

I will use some of my 210gr Bergers and go to the lands.

I calibrated based off listed loads, absolute perfect pressure readings are not really needed to compare. Notice the 3,000 psi extreme spread of the same loadings it would be pretty hard to get exact numbers. A 175gr SMK getting 2750fps out of a 20" bbl matches up pretty good in QL to be 71,000psi as do my other test including the factory Hornady Match.

Same issue with chronographs and, in a different way, digital scales at grain weights . . . neither can be calibrated by the normally equipped user.

What's the difference, are you saying at a certain pressure the strain gage reading is is somehow less reliable than it was 10k psi ago, or that for safety's sake you want to know for certain what the pressure is? The intent of PressureTrace is not to run at max safety pressures, it is to compare one load to another and observe the ignition process. As always loads need to be worked up.

It is calibrated to specific installations by using known load data and chamber/bbl/case dimensions, or if I wanted I could send my handloads for testing and adjust accordingly, but yes it still wouldn't be calibrated to my exact rifle perfectly.

Next batch loaded up with 210gr Bergers. On lands, .005" off lands, .015" off lands, .025" off lands, etc. to .400" off lands. COAL go from 2.775" to 3.175"; actual numbers are gotten from base to ogive measurements. Hornady 168gr match cartridges are on each end for reference.

Tag

The numbers in chart #1 appear to be normal.  Rifle cartridge bullets typically have a seating depth where the peak pressure is minimized.

The existence of the minimum pressure point was explained in the 1960's by physicist Dr. Lloyd Brownell . When a cartridge fires, the brass expands at the neck and releases the bullet. It has to be accelerated forward to meet the throat, which takes time, during which the lower mass gas accelerates more easily and bypasses the bullet and starts leaking into the bore. (In super slow motion films you can see this gas precede the bullet exit at the muzzle, usually kicking some unburned powder particles along with it.) That gas bypass stalls the rise in pressure in the case just as bullet movement causes expansion of the powder burning space to commence. The more gas bypass there is, due to deeper seating, the later and lower the peak pressure in the chamber will be, due to that stall in pressure rise giving a head start to expansion. But only until that pressure minimum point in seating depth is reached.

The reason for the minimum is that there is a counter influence at work: as you seat deeper the powder starting burn volume is decreasing. More gas packed into a tighter space means higher pressure, and that increases burn rate and that causes faster rate of rise in pressure. As a result, if you seat deeply enough, that faster rise in pressure while the bullet is still in the case overwhelms the effect of the stall in pressure rise caused by the gas bypass effect. In other words, either side of the minimum pressure bullet seating depth, the shrinking powder space effect and the gas bypass effect trade off which one is the dominant term in determining peak pressure.

It's a long read but may be of some helpful insight.   Be sure to check out pages 47 and 48.

http://deepblue.lib.umich.edu/bitstream/handle/2027.42/3866/bac6873.0001.001.pdf?sequence=5


 I see that you started your test .030 off the lands, curious why you did not start your test at the lands.  If you would have stared at the lands and plotted a chart it most likely would have looked like Dr Brownells Chart with contact to the lands having  the highest pressure then decreasing in pressure in till you reached the pressure minimum.  



.330 off the lands, that is extremely deep, was the case mouth past the ogive at this point or does this 700 has a modified or worn out throat?

Great thread, very interesting.

I'm curious what would happen in situations where the bullet is bottomed out against the powder. This seems like it would be fairly common in semi's if not properly sized. I have seen it in pistol ammo when shooting plated bullets that were severely undersized..

Also, do you care to share some pictures of your setup (bullet trap, magnetospeed and pressuretrace). Thanks!

Update in OP with 210gr Bergers touching lands.

There are loads that are compressed but with my powders the pressures start getting high at 95% load density. My 210gr .308win  load that I am near completing is 3.090"coal, it would be compressed at 2.920" and would run about 80,000psi.

I hope you still have your brass from your first post. For the 175 SMK with pressures from 49K to 71K please post pictures of the primers and measure the case head expansion (CHE). I have read other posts about primers and CHE. The result was it is not directly related to pressure, and no way to reliably tell when you have crossed from the safe to the unsafe pressure level. I would like to see your results on primers and CHE.

This is something we've been doing in the 6.8 community for years, proven by pressure trace test data, along with the testing of different loads / chambers / rifling designs.

Always good to have another experiment to point to.

 Interesting how your Berger data differs from Quickload data.

Quickload says to add 7200psi to the start pressure when the bullet is touching the lands.

Depending on the cartridge and the load this usually increase chamber pressure upwards of 10K psi.

Wonder why your test loads differ so much from Quickload, Barnes, Lyman and yhe UIniversity of Mich?

Barnes pressure tested data.
*Note: beginning on the left with shot No. 1, distance is closest to the lands, and ends with shot No. 5 on the right, which was seated at the greatest distance from the lands. This applies to all charts.




Old Lyman manual.



University of Mich.



...And this from Shootingsoftware.com.

Might want to read this on PRE and CHE.

https://www.shootingsoftware.com/ftp/dbramwell%20july%2019%2004.pdf

Added to OP

Thanks! That was one of the works I was thinking of!

Thanks for the extra information!
All the primers look good to me, the CHE is minimal and those swipes look very minor. This confirms my opinion; without pressure equipment we are loading blind (until we are way over pressure).

Mistake.

The one primer is flatter if you look close. This is 7.62x51 brass so it may be more resistant to expansion in the heavy web area.

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Here is a quote from one of your previous threads.



How do you explain the contradictions between your two sources of pressure data.

In chart #1 you had a decrease in pressure between T-1 and T-2 with a Reduction in OAL of .050.  Reducing OAL by .050 reduced pressure, QL would had shown an increase.

In your Berger data chart you showed a Decrease in pressure from T-3 ( .015) off and T-5 (.035) off. In this example you again showed a Decrease in pressure with a Decrease in OAL of 25 Thousands.  QL would have shown an increase in pressure here.

Also in your Berger data you showed lower pressure touching the lands than at .015 off and greater.  By following QL recommendations on "Start" pressure while touching the lands, you would see an increase in pressure touching the lands by approx. 10,000 psi.

Maybe you should post the load densities, ES's of pressure they get from group of standard loads, powder, bullets, etc. from those three pictures you are married to and I could try to help you understand. You're like Algore with his hockeystick chart.


My results were not what I expected either but wont I repeat the test until I get the result I want. I thought there would be a slight increase in pressure at the lands but not more than 4 or 5000 due to the added case volume.
I was certain the increase in psi at the shortest seating depth would be double that of the one at the lands. Didn't happen that way likely due to load density.

Now if I develop a good pressure load say .060"  from the lands then I think will see an increase when when that load is put into the lands.

As to what manufacturers say, they are going to pos t "safe" information. They may only 1 out of 100 times(I am sure it is more than that) where there was a pressure increase at the lands, well they are not going to take a chance on that one percent.

The simple fact is, a longer COAL alone will decrease pressure due to more case volume more case volume means less pressure in the case and on the powder, less pressure on the powder means a slower burn rate, slower burn rate for the same charge will mean a lower peak pressure. This will be true until you get to the resistance of the bullet swaging into the rifling.

Look at the rise times supplied with the charts, they are an indication of burn rate, the bigger the number the slower the rate. I may need to add it to the first one, I am on my phone so I'll do it later.
I want to "bold" some text and add a smiley or 2 as well.

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The extreme spread of factory ammo PSI, is over 3,000psi so small changes are something that cannot hang your hat on. You have to look at the overall curve.

I cannot see the charts on my phone, I'll try to answer when I get home.

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...but this is not what your Pressure trace data shows.

In the first chart you reduce OAL in T-1/ T-2 from .030 to .080 off the lands.  This reduction in OAL by .050 Reduced pressure, it did not increase it.

In your Berger data your reduced the OAL T-3 /T-5 from .015 to .035 off the lands.  This reduction in OAL of .020 also reduced Pressure and did not increase it.

Your own Pressure trace Data disagrees with your above quote and also the quote below.

Look at the bold conjunction in that sentence you posted out of context dug up from another thread. Had I known my every word was going to be scrutinized I would have been more specific in my wording. And then in my above quote you cut out a very important part don't you, I'll put it back in.

The extreme spread of factory ammo PSI, is over 3,000psi, so small changes are something that cannot hang your hat on. You have to look at the overall curve. You are pointing out small insignificant amounts. Look at the reference loads I had, a 10 shot group of identical loads that had a 3k psi extreme spread.

Quickload does not know when your bullet has reached the lands until you tell it so.  At the point of contact with the lands QL says to add 7200psi to the "start" pressure.  Your Berger graph disagrees with QL on this point as there was no where near a 7200+ increase in pressure.



So, with a spread of over 3000 psi, your Berger "curve" from .000 off to .100 would pretty much be a straight line.  Then a slight increase in pressure at .175 off and another straight line from .175 to .325 off.

With a spread of over 3000 psi you basically had no change in pressure from .100 off( 55,108 psi)  to a whopping .325 off ( 58,917 psi).

What would Quickload calculate the increase in pressure with a reduction in OAL of .225?  I'm guessing about 10K psi.

QuickLoad is a computer model not empircal data. I use it all the time for case volumes and getting lenths and getting an idea of what happens when certain items are changed and for getting charge information. It is often very close on specifics as far as velocities but in some cases it is not because it cannot take into account all the variables.

Burn rates vs.case volume vs. pressure are not going to change in at linear rate so you cannot say the .050 shorter coal equals 3k psi pressure increase. At 95% load density it will be faster than at 80% load density.
Look at the rise times those are micrseconds from 25% of peak to 75% of
peak the bigger number is a slower rate.


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 With all that is positive and impressive with Quickload it does have a few flaws.  One of these flaws is how it calculates pressure in regards to OAL.

The trouble lies in the "all or nothing" nature of QuickLoad's suggested adjustment.

It suggests adding 7,200psi when loaded to touch the rifling but makes no recommendation for loads OALs that get closer and closer.

In reality, there should be a small increase every time the OAL is made longer.

This is a great post!   Thank you

You would also think the diameter of the bullet touching the lands "ie the contact area" would affect this pressure change.
Smaller bullets might be 7200 but would a .416 bullet have the same pressure change loaded to contact as a .223? Seems to me the larger contact area of the 416 would raise pressures more.

Why don't you cite your quote, because it is from some guy on another forum?


"Fooling around with alternating current is just a waste of time. Nobody will use it, ever."
-Thomas Edison, 1889

"Heavier than air flying machines are impossible."
  -- Lord Kelvin

"There is not the slightest indication that nuclear energy will be
obtainable."
  -- Albert Einstein, 1932


"Rail travel at high speed is not possible because passengers, unable to
breathe, would die of asphyxia."
  -- Dr. Dionysus Lardner, 1793-1859


Even smart guys get it wrong sometimes.

http://thefiringline.com/forums/showthread.php?t=549077

I think you would be impressed with the knowledge of both Brian and Unclenick.  You should check in and pick their brain as far as Quickload is concerned.  That's not to say you are not knowledgeable, you know more about QL and Pressure Trace then most, especially me.  Check them out, you may even enjoy it.

Update in OP, more test touching the lands and no increase in pressure.