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Old April 1, 2013, 09:19 AM   #1
jim8115
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Pressure Vs Velocity

Supposing you have a load with published, safe data. In this example a 180 gr cast for a .40 S&W. If you need to substitute a different profile ( but same weight ) bullet that may be longer, or need to seat deep to accomodate a certain gun , is it safe to assume that as long as the velocity is within the published range that the pressure is also? Assuming same powder , of course.

In other words, if I have a load that is 900 FPS according to the book, and verifies with a chrono, and I change bullets / seat deep, drop the powder charge to allow for it, and it is still 900 FPS, should the pressure be the same?

JIM
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Old April 1, 2013, 10:43 AM   #2
SL1
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I don't have the time to run QuickLOAD right now, but I will do that for you, later.

My experience tells me that reducing the powder space and adjusting the charge weight to get the same velocity will probably give you somewhat higher peak pressures, but I don't know if it is enough higer to matter.

On the other hand, reducing the powder charge to give you the same percentage of fill of the powder space will probably give you somewhat lower velocity and about the same peak pressure (maybe a tad lower pressure).

When I get a chance, I will run some examples.

The magnitudes of the changes will depend on the powder being used, with relatively fast powders (for the particular applicatioin) making bigger differences. So, it would be best if you also told us what powder(s) and bullets you are thinking about using and comparing.

Also, it is important to point out (to others) that you need to use a chronograph (as you said that you are) to measure the velocities from YOUR gun, using both your handloads with the same components as the published data and your handloads with the substituted component(s). Trying to duplicate the PUBLISHED velocities with handloads in your gun that do not match the components used in the published data is a high-probability way to get into trouble.

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Old April 1, 2013, 11:22 AM   #3
Rimfire5
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Remember, the bullet shape and amount of bullet in the neck and into the cartridge body can change the pressure so you can't assume anything until you ensure that the two bullets are very close in characteristics.

Seating the bullet out into the rifling and away from the powder in the cartridge body can also change the pressure.

I would recommend that you look up the powder limits for each bullet, even if it is the same weight to ensure that you are not overloading or potentially compressing the powder.

If you were in the middle to low end of the load table with a bullet with like characteristics you should be OK on pressure, but if you are near the limits, you might be surprised by a long bullet that is seated back further into the neck than you thought.

For example, the 168 Sierra Match Kings and the Nosler 168 Custom Competition bullets for my .308 look very much the same but the ogive position is different and the Nosler is a bit longer in the tip. Seating them both at the same OAL causes the Nosler to be 0.020 further back into the neck than the Sierra. That could increase the pressure.
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Old April 1, 2013, 01:26 PM   #4
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Rimfire,

Jim is asking about .40 S&W with a 180 grain bullet in a pistol. That is about as far as it can get from a .308 Winchester in a rifle, at least with respect to how the various internal ballistics parameters will be important or unimportant to his question about matching pressures by matching velocities.

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Old April 1, 2013, 02:11 PM   #5
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QuickLOAD calcs for .40 S&W with 180 grain bullets:

Load adjusted to 900 fps with relatively short 180 grain bullet, then variations made using longer bullet of same weight to get (a)same velocity, (b)same % case fill, and (c) same pressure. Assumed 3.83” barrel length and 1.135 loaded cartridge length for all examples. Using Bullseye powder. Other parameters, including case capacity, are the default QuickLOAD values that may tend to over predict pressure and velocity until adjusted to match the cases and guns used in the pressure-tested data. However, the TRENDS AND RELATIVE MAGNITUDES OF THE CHANGES in the results should serve to illustrate the effects being discussed. [Note, I had originally selected the powder to give a peak pressure in all examples that was just below the SAAMI limit of 35,000 psi for this cartridge, but goofed in picking the longer bullet from the data table, getting the Hornady 200 grain XTP-HP bullet instead of the 180 grain bullet. After finding the error and making the corrections, the peak pressures are all much lower than exciting values for this cartridge. The differences in examples a, b and c would be more important if the peak pressures were higher, but I do not have the time right now to redo all of these examples.]

Code:
Bullet               Bullet      Charge      Fill    velocity     pressure
                     Length      Weight       %

Speer TMJ            0.630        4.25       77.5      902        23,325

Hornady XTP-HP       0.661        4.25       86.9      934        27,652
              (a)                 4.08       83.4      903        25,377  
              (b)                 3.79       77.5      851        21,807
              (c)                 3.92       80.2      875        23,360
Note that the longer bullet produces much higher pressure but not much higher velocity. Dropping the charge to the weight that gives the same velocity does not decrease the pressure by much, compared to the pressure with the shorter bullet. Dropping the charge to the same percentage of powder space fill drops the pressure back to just a little below the original pressure with the shorter bullet, but drops the velocity quite a bit.

So, in summary, you cannot control pressure when substituting bullets just by matching velocities with a chronograph.

However, you can do a pretty good job just by measuring the case capacity, subtracting the volume occupied by the bullet to get the powder space, and using the same ratio of charge weight to powder space when using different bullets (of the same weight and construction type) that have different lengths, or when just changing the seating depth for the same bullets from what was used in the pressure tested data.

Of course, changing bullet construction type is a whole ‘nother issue if the different bullets are have much different amounts of force needed to engrave them with the rifling. And, changing bullet profile so much that one of them is jammed into the rifling before firing is another issue.

It should also be pointed out that matching the % of powder space fill will give slightly HIGHER pressures when going to shorter bullets or shallower seating depths of the same bullets. So, don’t take this as an exact calculation that can be applied in either direction without the appropriate cautions.

Also note that all of the examples that I used resulted in pressures within SAAMI limits. But, if I had started with a max load for the short bullet, then the loads with the same charge weight and the same velocity could be way over the SAAMI limit.

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Last edited by SL1; April 1, 2013 at 02:52 PM.
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Old April 1, 2013, 04:07 PM   #6
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OK, I reselected the powder to still get 900 fps, but with a higher pressure initial load, still keeping all loads below SAAMI limits.

Powder is Titegroup:

Code:
Bullet               Bullet      Charge      Fill    velocity     pressure
                     Length      Weight       %

Speer TMJ            0.630        4.21       62.5      900        28,903

Hornady XTP-HP       0.661        4.21       70.1      924        34,367
                                  4.04       67.3      900        31,519  
                                  3.75       62.5      857        27,043
                                  3.88       64.5      876        28,992
As you can see, the same trends apply, but the pressures get closer to the limit.

Just to show what can happen with a load that is already at the limit, here is another example where I did not restrict the initial load to 900 fps, but rather selected it to be close to the SAAMI limit already. WARNING, SOME OF THESE LOAD EXAMPLES EXCEED SAAMI PRESSURE LIMITS AND ARE SUPPLIED SOLELY AS EXAMPLES OF THE EFFECTS OF CHANGING LOAD COMPONENTS OR PARAMETERS FROM WHAT IS SPECIFIED IN PRESSURE-TESTED DATA. DO NOT USE THESE RESULTS TO CONSTRUCT ACTUAL HANDLOADS.

Code:
Bullet               Bullet      Charge      Fill    velocity     pressure
                     Length      Weight       %

Speer TMJ            0.630        4.6        68.3      953        34,730

Hornady XTP-HP       0.661        4.60       76.6      979        41,578
                                  4.40       73.2      951        37,758  
                                  4.10       68.3      909        32,505
                                  4.24       70.6      929        34,887
Please do not try to interpolate between the constant velocity example and the constant % powder fill example to try to get an estimate for a constant pressure load. That would probably change its position between the two other examples as parameters like powder type, charge weight, bullet weight, bullet diameter, case capacity and other things change.

Just take this as a demonstration that matching chronograph velocities is potentially NOT a safe method when reducing powder space, while matching percent fill is a safe method for doing that. Again, matching percent fill of the powder space is less safe when INCREASING the space instead of decreasing it.

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Old April 1, 2013, 08:17 PM   #7
jim8115
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load info

Bullet -180 gr truncated cone .625 long
Powder - 5.0 grains Power Pistol
OAL 1.085
Chrono'd at 905 FPS from a 4" XD

JIM
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Old April 2, 2013, 07:26 AM   #8
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Jim,

I can use QuickLOAD to calculate the pressure for the load that you posted. But, by itelf, that doesn't help you, because it takes some tuning of the QuickLOAD prameters before the results of a calculation are reliably accurate for a given gun.

What is more useful it to compare two loads in the same gun using QuickLOAD. So, in your case, knowing what load you want to compare to the posted load would be helpful.

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Old April 2, 2013, 09:28 AM   #9
jim8115
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Actually, I have a copy of Ql. Im just not sure I trust it. In order to get it to match known loads I have chrono'd I have to change some of the paramaters. If i remember, for Power Pistol, I changed the Ba to 2.45. Then the velocity matches what I have checked. Doing this, my loads are fine. I just dont know that thr pressure numbers are accurate

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Old April 2, 2013, 10:39 AM   #10
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no

But, your 5.0g Power Pistol load is well below MAX.
Well below.....

So in your specific gun swapping 180g bullets won't matter pressure-wise in ANY significant way.
I mean you could run virtually any cast, plated, or jacketed 180gs with 5.0g.


In the interest of SAFETY, the standard approach is still always best: Start low work up slow. When changing ANY component or tooling start low and work up.
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Old April 2, 2013, 10:43 AM   #11
Brian Pfleuger
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I use QL constantly and NEVER "trust that the pressures are accurate". Having to change any number of parameters is a given. I particularly don't trust it for handgun loads. Very much of what it tells me simply doesn't make sense.

Even with rifle loads, where it can be remarkably accurate, I still don't give it much "trust" until I see numbers that match over the entire charge spectrum for a given combination.

I now also have RSI Pressure Trace and it doesn't agree with QuickLoad and neither of them agrees with published data.

They are helpful and interesting tools but you have to understand their limitations. At least with the Pressure Trace, I have a good sense that the RELATIVE pressure is consistent. If a load develops 45,000psi, it might not actually BE 45k but if another load also develops 45k, it's very reasonable to assume that they are "the same" in that respect. I like QuickLoad a lot but just because it tells me that two different powders will both make 45k, doesn't mean they will.

As mentioned by SL1, the numbers are helpful in a relative sense. They may not be "right" but what happens when you make changes is instructive, even if the absolute values are not helpful.
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Old April 2, 2013, 10:52 AM   #12
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"But, your 5.0g Power Pistol load is well below MAX.
Well below.....

So in your specific gun swapping 180g bullets won't matter pressure-wise in ANY significant way.
I mean you could run virtually any cast, plated, or jacketed 180gs with 5.0g."

Even at 1.085?
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Old April 2, 2013, 11:06 AM   #13
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The pressure does not matter.
The effects of pressure can matter.

So knowing the pressure may do no good.
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Old April 2, 2013, 11:14 AM   #14
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bullet vs other stuff

I hesitate to suggest a significant shortening of OAL.

But I have run 220g lead over 5.0g Power Pistol at 1.130".....
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Old April 2, 2013, 12:18 PM   #15
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I just messed-up a long post with graphs and lost it. I don't have time to try to reproduce it, now. So, here are a few words to give you part of the picture.

When I graph the pressure-tested manual data for velocity vs charge weights for a bunch of powders, SOMETIMES I find that I get a bunch of straight lines that are PARALLEL (or nearly so), sloping upward to the right. And, SOMETIMES, calculations with QuickLOAD produce lines with the same slope. With slight modifications to the Ba for each powder and ONE case capacity, I can pretty much match the data with QuickLOAD. In that situation, I tend to believe that QuickLOAD is modelling the data with good accuracy. I then try to see if my own chronograph data also fits the pattern. If so, then I THINK that the pressures from QuickLOAD are probabally pretty close to the truth. Data from the Speer and Hornady manuals for the 357 Sig is a good example of this type of situation.

But, often the data from the manuals for a bunch of powders produce lines that are NOT parallel. I don't know what that means. And, only a few, or even none of those lines may be the same slope as the lines produced by QuickLOAD. So, for the powders whose lines slopes don't match the QuickLOAD slope, there is NO way (that I have found at least) to change QuickLOAD parameters so that QuickLOAD results match ALL of the velocities for various charge weights of the powder. That situation tends to make me distrust QuickLOAD. The Hornady and Speer data for the .40 S&W are an example of this type of situation.

There is yet a third situation, where the line slope of published data doesn't match other published data, nor QuickLOAD, nor my experience, nor any rational understanding, to the point that I trust NEITHER the published data nor QuickLOAD. The .357 Magnum data in the Nosler manual is an example of this situation. (I tried to get some comment on this last example in a previous thread on this forum, but the responders did not seem to get the point of the post, and Unclenick did not respond.)

Anyway, with the .40 S&W example I discussed above, I don't think I would trust QuickLOAD for pressures UNLESS I was able to match MY OWN chronographed velocity vs charge weight information over a significant range of charge weights with the load I was considering for component substitutions or COL changes.

Still, as others have pointed out by now, QuiickLOAD does offer the best available way for handloaders to address the type of question that you asked. You can get some idea of the RELATIVE differences and MAYBE the RELATIVE SIZES of the differences from modifications to load parameters.

But, as for REAL pressure, when attempting to adjust QuickLOAD to match a set of chronographed velocity vs charge weight data, it is useful to note that you get DIFFERENT PRESSURES when you match with Ba and when you match with case capacity. And, there is nothing to guarantee that the REAL answer lies between those two results instead of outside of their span.

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