h4895 hodgdon load data 168 gr smk 41gr to 43.5, p-max says 63000 psi pressure at 41,2 grs this does not seem correct ,i have loaded and used 42.2 grs with no pressure signs but just started using p-max ballistic and there seems to be too high a pressure has any one found p-max pressure readings to be a long way off true pressure readings ? i am looking to get 2650 fps from my rpr ,1in 10 twist 20" barrel using h4895 and 168gr smk so around 43 grains would be required but not at p-max 66000 psi pressure !!
p-max pressure
- Thread starter ashray
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billgatese30
Well-Known Member
Pmax on the CIP spec is the safe published pressure. Your particular rifle may handle higher pressure without necessarily showing pressure signs however all that means is your rifle can do that, it doesn't indicate what pressure it would take for the rifle to fail or for symptoms of excessive pressure to appear. This is why its not typically advisable to exceed Pmax. To look at this from the other side of the coin, the CIP Pmax is the minimum pressure all rifles should safely handle within their design brief, although every manufacturer will have a safety factor designed in which is why they don't fail until significantly above Pmax, and the proof rounds used to proof the rifle confirm this at the time of proofing.
Pmax is really good, and pretty accurate I've found.
Who says your rifle will show signs of excess pressure at 63k?
There's no point in second guessing velocity - use a chronograph!
So... what is the cartridge?
Yes.... but even the Hodgdon pressures are pretty frisky.
Generally, internal ballistic models like this will do a much better job on predicting the velocity than predicting the pressure for a variety of reasons. That is why pressures are colour coded in P-Max to give you general indications regardless of the actual numerical value of the calculated pressure. All I can say is that if the pressure value is highlighted in red, then you really should be paying attention....
Scipio
Well-Known Member
Hello all
I was just playing around a little with p-max myself, (but only a little, so i might have missed something, it has to be said), but i did note that:- it only changed it's internal ballistics calculation for every whole grain of powder changed, so going from 25,0 to 26,0 for example or from 45,0 to 46,0, at which point it then jumped circa 15.000 psi.
If that is indeed the norm for p-max it could explain why it might not always mimic GRT or Quickload super closely, right?
- it doesnt utilize a specific bullet choice option for each calculation, but instead broad genre one, and that too i'd guess will likely go to make it's calculations a bit more coarse than a program that correctly takes that into consideration.
PS. when p-max writes "bore riders with drive band" , do they mean modern monos, like barnes ttsx, hornady cx, fox classic etc?
That is not what it is supposed to do and (I just tried it) that is not what happens. You can make as small a change as you like and it will respond accordinglyHello all
- it only changed it's internal ballistics calculation for every whole grain of powder changed, so going from 25,0 to 26,0 for example or from 45,0 to 46,0, at which point it then jumped circa 15.000 psi.
No.... results (for velocity anyway) are usually pretty close to GRT. I have to admit, I don't have an up-to-date version of QuickLOAD.If that is indeed the norm for p-max it could explain why it might not always mimic GRT or Quickload super closely, right?
. Results for pressure differ much more between the models. But all the different models make approximations to achieve generality and they are generally 'tuned' to give good results for velocity rather than pressure. Pressure can change quite significantly from gun to gun for the same load, even though the velocity is pretty much the sameI would not agree. All an internal ballistics program cares about is the weight of the bullet and the friction of the bullet in the bore. GRT and QuickLOAD need to know the shape of the back end of the bullet as well in order to calculate the powder space from case capacity. But since P-Max asks for the powder space directly, there is no knowledge of the shape of the bullet needed. As for bullet friction, you get more choice than any other ballistics program...!
From the P-Max manual...
"Bore riding' bullets, (as made by Nielsen and Peregrine for example), are becoming more popular these days. They are generally turned from solid brass or bronze and the main bullet diameter is actually (or near to) the bore diameter of the barrel. They have thin drive-bands which are engraved by the rifling and seal the bullet in the barrel, so the high pressure gasses behind the bullet do not leak past. The shot-start pressure and the barrel friction of these bullets is less than conventional bullets, so that a faster powder can be chosen than would be appropriate for a copper jacketed bullet of the same weight.
Monolithic solids which are full groove diameter and don't have drive band grooves on them, (as made by Barnes and Woodleigh for example), are harder to engrave and so require a higher shot-start pressure than conventional copper jacketed bullets. They will also have a higher friction force in the barrel."
Scipio
Well-Known Member
Hello Borbal and thank you for the informative answersA.
That is not what it is supposed to do and (I just tried it) that is not what happens. You can make as small a change as you like and it will respond accordingly
B
No.... results (for velocity anyway) are usually pretty close to GRT. I have to admit, I don't have an up-to-date version of QuickLOAD.
C.
I would not agree. All an internal ballistics program cares about is the weight of the bullet and the friction of the bullet in the bore. GRT and QuickLOAD need to know the shape of the back end of the bullet as well in order to calculate the powder space from case capacity. But since P-Max asks for the powder space directly, there is no knowledge of the shape of the bullet needed. As for bullet friction, you get more choice than any other ballistics program...!
D.
From the P-Max manual...
"Bore riding' bullets, (as made by Nielsen and Peregrine for example), are becoming more popular these days. They are generally turned from solid brass or bronze and the main bullet diameter is actually (or near to) the bore diameter of the barrel. They have thin drive-bands which are engraved by the rifling and seal the bullet in the barrel, so the high pressure gasses behind the bullet do not leak past. The shot-start pressure and the barrel friction of these bullets is less than conventional bullets, so that a faster powder can be chosen than would be appropriate for a copper jacketed bullet of the same weight.
Monolithic solids which are full groove diameter and don't have drive band grooves on them, (as made by Barnes and Woodleigh for example), are harder to engrave and so require a higher shot-start pressure than conventional copper jacketed bullets. They will also have a higher friction force in the barrel."
A.
I just tried it again, and indeed you are right! I dont know what happened the last time, but i screenshoted it. Very strange, but i must be humble enough to admit that it was more likely my fault, than the programs.
B.
C.
Aha, very interesting! I'd have thought GRT and QL also took into account the difference in band structure/baring surface of each projectile type, but if they do not, and it's mainly a question of knowing the shape of the back end of the bullet in order to calculate usable case capacity, as you describe, then i guess p-max actually offers a more universal tool than the other two, allowing to also use bullets not present in a set database, as long as one measures the usuable case capacity as asked by p-max. Very cool!
D.
Aha, i see, but is that still the case, or was it primarily the case with the early generations of barnes etc but not so much the new ones? Because i thought that they'd developed their projectiles to exactly lower those intial higher pressures that the early gen monos produced? But i might be wrong of course.
This video speaks a bit about it. One of the approximations in GRT/QL is that they do not have bullet friction as a separate variable. Instead, they increase the projectile mass by about 4% to mimic the effect of bullet friction. However, this friction loss then goes as the square of the velocity, whereas it should be independent of velocity. P-Max does have friction as a separate variable.
D.
Aha, i see, but is that still the case, or was it primarily the case with the early generations of barnes etc but not so much the new ones? Because i thought that they'd developed their projectiles to exactly lower those intial higher pressures that the early gen monos produced? But i might be wrong of course.
There is a difference between 'bore riding' solids and 'bore diameter' solids, as I have tried to explain.
Scipio
Well-Known Member
Hello again Borbal
A.
Well i do think i am somewhat onto the concept of there being at least two different ways of ensuring the seal to the barrel as well as controlling how large the bearing surface of the mono bullet becomes in the barrel.
So far so good.
What i do hope you can help to enhance my understanding about is:
- how does p-max takes into consideration the development of design solutions in the barnes style of mono path, that i believe that barnes, and other brands , have had over the years?
As i assume that development might well effect the amount of both the shot-start pressure as well as the friction force in the barrel, no?
- And the same goes for the possible variance on the same two areas between different makers of monos of that genre.
basically what i am curious about is, how p-max manages to calculate accurate (enough) pressure predictions, if it doesnt take into account the difference in designs of the different bullet makers, or the same makers, but ones who have refined and changed their designs over time? It is an honest question, as i am genuinely curious about it
.
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How important is the difference in design of the bullet to the final maximum pressure and muzzle velocity? It is a big enough deal that I have identified four different bullet types which will behave significantly differently in terms of shot start pressure and bullet friction and which encompass most (if not all) the different bullet construction types available. It is up to you to decide on which of these four categories your bullet of interest applies.basically what i am curious about is, how p-max manages to calculate accurate (enough) pressure predictions, if it doesnt take into account the difference in designs of the different bullet makers, or the same makers, but ones who have refined and changed their designs over time? It is an honest question, as i am genuinely curious about it
Scipio
Well-Known Member
Hello again @borbal .-)
Once more thank you fo answering, and, not least thank you for p-max! I didnt realise i was communicating with one of its creators. Having free tools like these, with guides of use freely available as well, is very much appreciated!
A. Ok, i guess that answers that.
Hello @takbok , and thank you for chipping in as well!
- Very interesting to hear! I shall be loading fox classic hunter 100 grn and hornday cx 120 grn in 6.5x57 and 130 grn fox classic hunter in 7x57.So I shall try and keep and eye on things, and compare the different online rescources and their predictions vs what then comes up when testing velocity wise. It shall be interesting to see how these 3 load developments fare, in this context.
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