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New BBT "P-Max" internal ballistics program


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That’s a very useful update Geoffrey - thank you have to say I like the way the program works for me 

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3 hours ago, Ronin said:

That’s a very useful update Geoffrey - thank you have to say I like the way the program works for me 

I am very pleased to hear that. How are you estimating "usable case capacity"?

The next update will be to enlarge (greatly) the table of cartridge usable case capacities so people can make a stab at the performance of different cartridges. 

Best wishes

Geoffrey

 

 

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Hi Geoffrey

 

seat the chosen bullet to depth using an empty unprimed case then fill with water with syringe 

Weigh before h2o insertion using upturned case in a child’s play dough (small pot and cartridge nose down ) 

Zero scale - insert h2o with syringe  ans hypodermic needle and once filled to base of primer pocket weigh again 

This gives reasonably accurate case capacity (useable case capacity) 

 

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Thanks for this very useful tool Blueboy

One thing if I may?  It does not differentiate between small or large primer size which is pretty significant when looking at .308 and 6.5mm cases.

I shoot 6.5CM and the load for SRP Brass is very different for LRP brass to generate the same pressures, this mostly due to the different ignition energy of the primers in relation to the powder column.

For example, if I use 47.5 grns useable case capacity and enter my bullet weith of 139 grains, using RS62, it should be close to 98% fill with 44gr RS62 (small primer brass) and result in a 24 inch barrel of close to 2700fps.  Your model predicts 2658 which isn't too far off but that's my data using SRP brass for 2705fps (20 degrees).  You wouldn't want to go there pressure-wise with LRP brass and a large rifle primer!  From actual data, 42grns RS62 under the same bullet at the same OAL using LRP brass gives closer to 2750fps and 42.5 grns gives just over 2800fps.

There appears to be something missing from the model in terms of assessing pressure/velocity as this must account for primer ignition energy and bullet seating depth.

Having tried it with all my loads it does  seem to be adrift for my 6.5 and .308 loads (under-estimates velocities by quite a bit but in .308).  Pressures modelled seem to be high compared with velocities shown for Vhit powders, and velocites significantly lower for RS powders than I achieve using RS50,  but bang-on for my 0.223 loads for all powders I use!.  In all cases I load with the bullet seated to the base of the neck.

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2 hours ago, Ronin said:

Hi Geoffrey

 

seat the chosen bullet to depth using an empty unprimed case then fill with water with syringe 

Weigh before h2o insertion using upturned case in a child’s play dough (small pot and cartridge nose down ) 

Zero scale - insert h2o with syringe  ans hypodermic needle and once filled to base of primer pocket weigh again 

This gives reasonably accurate case capacity (useable case capacity) 

 

further to that method;  a few thou of difference in insertion in, say, a .308 is a tiny variation in volume. Assuming the boat tail is pushed down further, the 'cylinder' taking up the additional volume is 0.308" /2 x pi2 x length.  So for example an extra insertion of 0.010" would take up 0.0122 cc (case capacity is 3.65cc nominal).  That weighs 0.185gr water.  (I apologise for the mix of units but most people understand grains)

According to QL, a 10thou deeper insertion that would make a 5 fps MV change (all else being equal)

I suggest not to worry too much about tiny changes.

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Thank for your comments VarmLR

It is very difficult to account for different primers. People complain about this for QuickLOAD as well. In real life, some primers are 'particle rich'  and spray a jet of incandescent particles into the powder column. The radiant heat from the particles ignites the powder, usually at the base first close to the primer. Some primers are 'gas rich' and the powder ignites as the primer gasses condense on the powder kernels and so heat up the kernels by conduction. Models like QuickLOAD, GRT or P-Max have no way of modelling for this as they assume the burning rate of the powder is proportional to pressure and temperature never gets a look-in during the simulation. The simulation is started by assuming an existing start pressure of 800 psi or so and it takes off from there, which of course is some point after the primer has done its work.

OK on the other information. Yes, the pressures that P-Max generates are higher for the maximum loads than the Vihtavuori manual shows. But it is invariably the case that the pressures indicated by P-Max are 'green' for starting loads and 'orange' or 'red' for maximum loads. I tried to tune the powder model for velocity agreement with the manual, not for pressure. I would be more troubled if P-Max underestimated the pressures as people might assume a load was safe if the indicated pressure was lower than it should be.

Generally, I am pretty happy if the velocities are within 100 ft/sec as variations in individual firearms will easily give a spread of 100 ft/sec for the same cartridge and I am not going to fight too hard to improve on that. However, other feedback I have had would suggest that in general there is a bias to underestimating velocities and if the feedback continues to suggest that, I will take a look at fixing that.

Thanks again for your comments

Geoffrey Kolbe

 

 

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Thank you Geoffrey.  Yes, I understand and once the case capacities have been measured, your load estimates don't seem to be too far off and certainly within 100fps of wehat I am getting, but as you say, err on the low side for MV.  I have found your stated pressures a bit on the low side for some loads V's velocities.  The small primers , especially with the small flash hole variety brass can have a very different pressure rise characteristic I think than standard large rifle primers when used with large rifle loads (specifically the 6.5CM/.308 palma brass).  Load development though always trumps modelled predictions which I treat as a good way of perhaps comparing powders and establishing starting loads.

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1 hour ago, VarmLR said:

 Load development though always trumps modelled predictions which I treat as a good way of perhaps comparing powders and establishing starting loads.

Could not agree more.

Load development should be done on the range with your rifle, not on a keyboard in front of a computer screen

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Played with PMax a little yesterday and today have shot the same loads “live”

Case is 260 improved 

Bullet 150 Smk

Shot five loads 

 

All came in within 40 FPS of predicted using PMax

Three gave  accuracy in the low .2’s 

 

Very pleased 

 

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Rs 60

I also spent a little time measuring available powder space h2o capacity and used an average value which appears to reduce the actual vs predicted difference 

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Indeed, I have plans to include as many powders as possible! Unfortunately, powder companies are very reluctant to part with their closed-bomb data on their powders and most companies do not even bother to reply. I am planning on building my own closed-bomb so that I can do my own analysis of powders, but even so it will be a marathon rather than a sprint to expand the P-Max powder libraries.

Thanks for your interest!

Geoffrey

 

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Hi Geoffrey,

I have Inputted exactly the same info on 2 separate days and had 2 Considerably different results. Have you updated something in the last few days?

its with viht n550
 

the first result was pretty much bang on for velocity and expected pressure for my current load, it matched very accurately to the software, however tonight it’s way off 

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20cal 

41gr case capacity 

36.6gr of N550 

39gr bullet

26” barrel

1.902 case Length 

first time I used it predicted 4206fps at 52595psi - very close to my findings with the chrono 

I tried again last night with the exact same data input and got 

4378fps for 59708psi 

it did predict the load density at 95% both times but all other results were different 

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Yes, I did slightly adjust the burning rates of all the powders listed. The reports I was getting back was that velocities were on the low side ranging from spot-on to about 100 ft/sec slow. Then I figured most handloaders actually load out to touch the lands whereas the data I had used to calibrate the powder models was from reloading manuals, where they load for a standard COAL and not to touch the lands, which would lower the pressures and the muzzle velocities.

The model assumes a shot start pressure which has to be reached before the bullet sets off down the barrel, which is equivalent to loading out to touch the lands, so I figured I would just increase the burning rates slightly to try and reproduce that scenario. I may have overdone it slightly😔

If you look at the bottom of the front page on the website, where you enter the data for the first time, you will see a version number for the simulator. If you click on that it will take to a listing of the dates and what was updated on the simulator.

Were you loading out to touch the lands?

Thanks.

Geoffrey

 

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Thanks very much for your comments. I think in retrospect,  a 'global warming' of the burning rates was a mistake.  I should have attended to the burning rates individually. I will remove the program temporarily while I review the burning rates. It should be back online in a couple of days.

Thanks for your patience. 

Geoffrey

 

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P-Max Internal Ballistics simulator is now back on line. The simulator has been improved slightly and as a result, all Vihtavuori and Chemi-Swiss powders have been re-calibrated.

I look forward to hearing how it works for you now

http://www.bbt.scot/ballistics/pressure.htm

Thanks

Geoffrey

 

 

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4 hours ago, borbal said:

P-Max Internal Ballistics simulator is now back on line. The simulator has been improved slightly and as a result, all Vihtavuori and Chemi-Swiss powders have been re-calibrated.

I look forward to hearing how it works for you now

http://www.bbt.scot/ballistics/pressure.htm

Thanks

Geoffrey

 

 

Hi Geoffrey

Firstly thanks for taking the time to develop this app, much appreciated.

I have run some calculations in Quick-Load and got very different results to your calculator;  my QL results reflect what I actually see ±20fps

Rifle 24" barrel

.308 155 Lap Scenar in Lapua brass.  43.5gr Viht N140.  I inputted my measured case volume with bullet inserted to my usual depth - this is 47.58gr water.

Why does it matter what the case length is if I have measured the case volume with bullet fitted?  Case length inputted is 2.005"

QL gives 53672 psi peak pressure and 2732fps Mv  99.97% combustion

P-Max gives 65241 psi peak pressure and 2680fps Mv  92% combustion

Any ideas please?

 

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38 minutes ago, Popsbengo said:

Hi Geoffrey

Firstly thanks for taking the time to develop this app, much appreciated.

I have run some calculations in Quick-Load and got very different results to your calculator;  my QL results reflect what I actually see ±20fps

Rifle 24" barrel

.308 155 Lap Scenar in Lapua brass.  43.5gr Viht N140.  I inputted my measured case volume with bullet inserted to my usual depth - this is 47.58gr water.

Why does it matter what the case length is if I have measured the case volume with bullet fitted?  Case length inputted is 2.005"

QL gives 53672 psi peak pressure and 2732fps Mv  99.97% combustion

P-Max gives 65241 psi peak pressure and 2680fps Mv  92% combustion

Any ideas please?

 

The P-Max simulator needs the case length so it knows the actual distance the bullet will travel. The muzzle velocity is actually a function of the actual distance the bullet travels, not the 'barrel length' per se. 

Comparing one simulator against another does not get us very far. Comparing P-Max against actual results is much more interesting.

Having said that, I note that the P-Max Mv is only 53 ft/sec. different that the QL result. Given that they are two completely different models and that gun-to-gun differences with the same load can easily be 100 ft/sec., I would call that a reasonable agreement. (Better than 2%)

As for pressures, I do not know the details of the QL model that Hartmut Broemel used. I think he got it from a book on ballistics put out by Rheinmetall in the 1970s, but that is in German so I cannot understand the model described in detail. The RS Chemie-Swiss reloading data does give pressures with its loads and generally the P-Max pressures would seem lower than they measure, but that is a very loose correlation of P-max pressures with quoted pressures and I suspect comparing the QL predicted pressures would be frustrating as well. P-Max pressures do seem very sensitive to loads approaching 100% loading density in a way that QL is not. For safety reasons, I would be more troubled if it was the other way around.

I have played around with Vihtavuouri powders for target rifle loads like yours and it would seem that N150 would be a better powder....? I know a number of shooters who use N150.

Thanks for your comments

Geoffrey

 

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5 minutes ago, borbal said:

The P-Max simulator needs the case length so it knows the actual distance the bullet will travel. The muzzle velocity is actually a function of the actual distance the bullet travels, not the 'barrel length' per se. 

Comparing one simulator against another does not get us very far. Comparing P-Max against actual results is much more interesting.

Having said that, I note that the P-Max Mv is only 53 ft/sec. different that the QL result. Given that they are two completely different models and that gun-to-gun differences with the same load can easily be 100 ft/sec., I would call that a reasonable agreement. (Better than 2%)

As for pressures, I do not know the details of the QL model that Hartmut Broemel used. I think he got it from a book on ballistics put out by Rheinmetall in the 1970s, but that is in German so I cannot understand the model described in detail. The RS Chemie-Swiss reloading data does give pressures with its loads and generally the P-Max pressures would seem lower than they measure, but that is a very loose correlation of P-max pressures with quoted pressures and I suspect comparing the QL predicted pressures would be frustrating as well. P-Max pressures do seem very sensitive to loads approaching 100% loading density in a way that QL is not. For safety reasons, I would be more troubled if it was the other way around.

I have played around with Vihtavuouri powders for target rifle loads like yours and it would seem that N150 would be a better powder....? I know a number of shooters who use N150.

Thanks for your comments

Geoffrey

 

Thanks Geoffrey.  With regard to bullet travel wouldn't overall length be more meaningful than case length as different 155 are quite different in length?

N140 gives me excellent results so I'm unlikely to try N150 with 155gr.  I will however be developing a 185gr load with N150

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57 minutes ago, Popsbengo said:

Thanks Geoffrey.  With regard to bullet travel wouldn't overall length be more meaningful than case length as different 155 are quite different in length?

No. The distance which the base of the bullet travels is what is important. It is not until the base of the bullet exits the barrel that the gasses stop accelerating it. (Actually, the muzzle blast continues to accelerate the bullet slightly even when the bullet has been 'launched'). The P-Max simulator actually assumes that the bullet is seated into the case by a length of one bullet calibre, which is why when you look at the graphs for bullet travel vs pressure and velocity, the graph starts a little before the 2 inches (for a 308 Win)

N140 gives me excellent results so I'm unlikely to try N150 with 155gr.  I will however be developing a 185gr load with N150

You would get an extra 100 ft/sec or thereabouts....😉 and no pressure problems...

Geoffrey

 

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