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borbal

New BBT "P-Max" internal ballistics program

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Tried comparisons of actual loads fired today in 6.5 and .223 and the powder burn rate re-calibration seems to have worked as in each case actual velocities were within 30fps for two 223 loads and to 6.5 loads but oddly a lot further adrift with a lighter .223 bullet where PMax over-estimated velocities by over 100fps.   I'd call that a good result overall though so feel a lot happier about using this.  What was a "safe" load previously in 223 is not a "red" load at 61Kpsi which certainly correlated to my blown primers today!

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6 hours ago, VarmLR said:

Tried comparisons of actual loads fired today in 6.5 and .223 and the powder burn rate re-calibration seems to have worked as in each case actual velocities were within 30fps for two 223 loads and to 6.5 loads but oddly a lot further adrift with a lighter .223 bullet where PMax over-estimated velocities by over 100fps.   I'd call that a good result overall though so feel a lot happier about using this.  What was a "safe" load previously in 223 is not a "red" load at 61Kpsi which certainly correlated to my blown primers today!

Well, that is gratifying - thanks very much for that

Interesting that a load which had previously shown up as "green" (under 51,450 psi) is now "orange" at 61k psi. What load was that...?

Thanks again for your comments

Geoffrey

 

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17 hours ago, Popsbengo said:

 

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?

 

I have been scratching my head about this for some while, but I think I have found the answer. 

It is of note that as the loading density (ratio) approaches 100% (full case) the P-Max pressures climb higher and faster than the corresponding QL pressures. But I also noticed that the muzzle pressures are lower in P-Max than in QL. So, what does that indicate?

It indicates that P-Max is treating the powder as more 'digressive' that QL. What does that mean?

When the powder is ignited in the case, there is not much volume as the bullet has not started moving up the barrel yet. So, you do not want the powder to burn too fast or the pressures could become catastrophic. However, once the bullet has moved off down the barrel and the volume has increased and is increasing quickly, then you want the powder to burn faster to keep pushing the bullet up the barrel. So, you ideally want powders to start burning slow and then get faster (generating gas more quickly) as the bullet accelerates up the barrel and the volume is increasing quickly. This is called a 'progressive' powder where gas generation gets 'progressively' faster.

Powders that have seven or even nine perforations (holes through the length of the cylindrical kernel) are common in large calibre cannons and such powders are progressive. As the powder burns from the outside and from the inside of all the perforations, the area of the kernel increases and so the gas production increases as they burn. Ball powders are 'digressive', because the area of the ball kernel decreases as it burns. But ball powders are usually coated with deterrents on the outside so that ball powders can be made to act like neutral or even progressive powders as they burn.

Cylindrical powders with one perforation (most rifle powders) and flake powders are usually classed as 'neutral' powders. As they burn, the area remains constant and the gas production stays roughly constant as the powder burns. But this is actually only an approximation which is only true for cylinders of infinite length and flakes of infinite width and length. However, this is an approximation used in many powder burning models for simplicity - including, I suspect, QuickLOAD.

In actual fact, the kernels in most rifle powders are only slightly longer than they are wide, and so they are not 'neutral'. These powders also burn from the ends of the cylinder and this cannot be neglected in 'short' cylinder kernels like in rifle powders. These powders are actually somewhat 'digressive', and that is how they are treated in P-max.

The result is that in QL, where cylinder kernels with one perforation are assumed to be 'neutral', the peak pressures will be lower because the powder is not assumed to burn from the ends and so gas is not generated as quickly as the kernel starts burning. However, muzzle pressures will be higher because the burning area of the powder kernel stays bigger for longer - because the kernel is assumed not to be burning at the ends.

I have run P-Max with the burning of the kernel ends turned off, and the results then start to look more like QL results, with lower maximum pressures, higher muzzle pressures and the powder being 'all burnt' sooner.

Anyway, I leave you to decide which model treats the powder burning more correctly...

Geoffrey Kolbe

 

Edited by borbal
bad english

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5 hours ago, borbal said:

Well, that is gratifying - thanks very much for that

Interesting that a load which had previously shown up as "green" (under 51,450 psi) is now "orange" at 61k psi. What load was that...?

Thanks again for your comments

Geoffrey

 

It was a very hot one!  About 23.9gr N130 under a 60gr Vmax.  Shot this load for 5 years without any issues or any eviden pressure signs such as flattened primers or marks on the case heads and it's shot really well, but recently I've had two batches of Vhit where MVs seem to have jumped by over 100fps using this load so I'm backing way off and re-developing the load to another accuracy node a grain or more down the load ladder.

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1 hour ago, VarmLR said:

I Shot this load for 5 years without any issues or any eviden pressure signs such as flattened primers or marks on the case heads and it's shot really well, but recently I've had two batches of Vhit where MVs seem to have jumped by over 100fps using this load so I'm backing way off and re-developing the load to another accuracy node a grain or more down the load ladder.

Are you using different cases as well? Sounds like you are pretty much filling the case and a slightly heavier case would make a lot of difference in peak pressure.

 

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I tested my case volumes when I went from Lapua to Sako...they were within half a grain, give or take a little.  It's a slightly compressed load and they're seated to book COAL but could drop the pressure off by seating them out a little as my chamber's been reamed to accept 80grn heavies.  I've found that power manufacturer's COAL (which they use in testing) usually results in good loads, but I'll play safe and knock at leats a grain to a grain and a half off my current loads as I only require MVs for the distances I shoot to be around the 3,000fps mark in these or a little less in boat tail equivalent weights.

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The Vihtavuori manual does not give data for the Vmax, but it does for the 60 grain SP by Hornady. The COAL was 57.0mm which gives a  calculated usable case capacity of 27.5 grains of H2O. The maximum load in the manual was 23.1 grains of N130, which should leave the case about 96% full (with the bullet seated). The quoted MV was 3173 ft/sec. P-max predicts 3197 ft/sec. with a warm peak pressure of 62717 psi.

23.9 grains of N130 gives a loading density of 99%, so a full case, and an MV of 3305 ft/sec., but the P-Max pressure is now distinctly "red" at 70776 psi. 

It looks like P-Max agrees that reducing your load or increasing your case capacity would be a good move....

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Yes.  My actual velocities averaged 3224fps with a few higher than that 3244fps so 61fps under predicted but close.  Previous load data shows velocities at 3150fps with an SD of 5 and a low ES as well, so lower pressure (same cases).  Batch to batch variations in powder could easily account for the change.  Lesson learnt the hard way and one gas cut bolt face later.  Thankfully still useable!

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45 minutes ago, VarmLR said:

Yes.  My actual velocities averaged 3224fps with a few higher than that 3244fps so 61fps under predicted but close.  Previous load data shows velocities at 3150fps with an SD of 5 and a low ES as well, so lower pressure (same cases).  Batch to batch variations in powder could easily account for the change.  Lesson learnt the hard way and one gas cut bolt face later.  Thankfully still useable!

Length of your barrel...? The Viht data was with a 620mm (25 inch) barrel.

Edited by borbal
missed information

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2 hours ago, borbal said:

The Vihtavuori manual does not give data for the Vmax, but it does for the 60 grain SP by Hornady. The COAL was 57.0mm which gives a  calculated usable case capacity of 27.5 grains of H2O. The maximum load in the manual was 23.1 grains of N130, which should leave the case about 96% full (with the bullet seated). The quoted MV was 3173 ft/sec. P-max predicts 3197 ft/sec. with a warm peak pressure of 62717 psi.

23.9 grains of N130 gives a loading density of 99%, so a full case, and an MV of 3305 ft/sec., but the P-Max pressure is now distinctly "red" at 70776 psi. 

It looks like P-Max agrees that reducing your load or increasing your case capacity would be a good move....

70776psi!! wow almost a proof load , That requires backing off the powder !Does the published data show this charge weight? Do you weigh each charge? Mixed cases can cause trouble, I would rework the whole load for safety.  

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34 minutes ago, KABOOM said:

70776psi!! wow almost a proof load , That requires backing off the powder !Does the published data show this charge weight? Do you weigh each charge? Mixed cases can cause trouble, I would rework the whole load for safety.  

Errrrr, yes, no, yes, no mixed cases involved and yes! 

I've been reloading for years, shooting LR for years and slipped up on this one load more because of an adjustment to seating depth from the original load taking up effective powder volume and a batch to batch increase in powder energy.  Having a hot load to start with, shaving off around 20 thou on seating depth and using a slightly hotter powder resulted in primers blowing.  I'll be reworking the load for about a 42 to 42.5grn charge which is close to another accuracy node.  The irony is I've been suing this charge for the past 4 years and only had an issue once before when using 20 thou primer cups rather than heavier cups needed for 223 hot loads.

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Major edit and an embarrassing admission.  It was not N130 (not even I would have loaded that far over book!) it was N133.  Max charge for a hpbt was listed as 24.7gr so I was well below this on my loads.  Redoing the Pmax calculator, it sows things in a more sane light, thankfully.  The primers blowing and the high MVs still point towards a fairly peppy powder batch just opened, so the lesson learned is re-do load dev for each now powder batch.  Batch to batch variations can be significant as I found out yesterday.

New PMax calc , 27.5hr H20 ccase capacity, 23.9gr N133, 26 inch 1/8 barrel:

56.4 Kpsi; 3159 fps.

Actual avareged MV over 10 shots was 3236fps.

Still a peppy load but nowhere near what I would have expected to blow primers.  I checked and the case heads are not bowed, and primer pockets are still nice and tight, so brass is A1 ok.  

Still, clearly over pressure so backing off to closer to 23gr seems sensible.

 

 

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All's well that ends well...

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Sort of...I still have to work out how a charge of 0.8 under book max resulted in blown primers.  The only theory I have for that is that the bullet bearing length, hence friction, is greater on the flat base 60gr Vmax than predicted for the hpbt.  That, in itself and especially for the 1/8 twist might account for the pressure difference along with a slightly hotter batch.  

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It might be worth investigating the throat of your chamber with a bore scope. I have seen "carbon rings" build up which are very difficult to remove and add quite a bit to he shot start pressure.You might think you clean your gun real good, but....

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Yes, agreed.  I did have a look two days back and the throat looks to be in remarkably good condition with no rings evident.  I clean out after each outing using KG1 Carbon remover and a brush before patching out, followed by WipeOut Tactical Advantage and patches for good measure.  I've found this regime far more effective the relying on the Wipe Out used on its own.

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On 6/10/2020 at 1:01 PM, VarmLR said:

Major edit and an embarrassing admission.  It was not N130 (not even I would have loaded that far over book!) it was N133.  Max charge for a hpbt was listed as 24.7gr so I was well below this on my loads.  Redoing the Pmax calculator, it sows things in a more sane light, thankfully.  The primers blowing and the high MVs still point towards a fairly peppy powder batch just opened, so the lesson learned is re-do load dev for each now powder batch.  Batch to batch variations can be significant as I found out yesterday.

New PMax calc , 27.5hr H20 ccase capacity, 23.9gr N133, 26 inch 1/8 barrel:

56.4 Kpsi; 3159 fps.

Actual avareged MV over 10 shots was 3236fps.

Still a peppy load but nowhere near what I would have expected to blow primers.  I checked and the case heads are not bowed, and primer pockets are still nice and tight, so brass is A1 ok.  

Still, clearly over pressure so backing off to closer to 23gr seems sensible.

 

 

Glad things are working out! My worst error was using data with one bullet and I used another with about twice the bearing surface.

At 32 degrees F it was ok the velocity was high enough to make me smile and I should have realized things were amiss. Sadly I didn't until summer. Stiff bolt lift, very flat primers made me look at things closer. After that shoot I disposed of all that brass and reworked the load with new brass and much less powder. Nosler developed data with the same bullet and powder yet even less powder, Im ok with that they have pressure testing ability I don't so I will use proven data from them. I have yet to blow a primer I hope I never do. The variables in dimensions, case capacity and lot to lot changes with powder make me very skeptical of things like Quik Load and any other calculated  un shot data. After re reading your total posts about this situation I think your def on the right track. Learning happens some times easy and harmless and others its HOLY BLEEP lets not do that again. Best wishes for good shooting.

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9 hours ago, KABOOM said:

Glad things are working out! My worst error was using data with one bullet and I used another with about twice the bearing surface.

At 32 degrees F it was ok the velocity was high enough to make me smile and I should have realized things were amiss. Sadly I didn't until summer. Stiff bolt lift, very flat primers made me look at things closer. After that shoot I disposed of all that brass and reworked the load with new brass and much less powder. Nosler developed data with the same bullet and powder yet even less powder, Im ok with that they have pressure testing ability I don't so I will use proven data from them. I have yet to blow a primer I hope I never do. The variables in dimensions, case capacity and lot to lot changes with powder make me very skeptical of things like Quik Load and any other calculated  un shot data. After re reading your total posts about this situation I think your def on the right track. Learning happens some times easy and harmless and others its HOLY BLEEP lets not do that again. Best wishes for good shooting.

Indeed!

The only oddity is I re read my load notes from 5 years ago when I first started using this bullet and no pressure signs were evident in any of the loads then and that was using CCI200 primers.  Rather that fight it, you listen to warnings like this, learn from them and move on.   The one major difference between then and now, is that then I used to neck size and now I FL size all the time.

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