Is there a direct relationship between velocity and pressure

[Downforce]

Hi

im just trying to figure out if I have a factory load of a given weight (say 28g) which is quoted at X psi/bar and Y fps

 

does that mean my 28g home load also measured at Y fps will be the same pressure as the factory load?

 

probably over simplifying with different wads crimps primers and powder but I read an article from a US professor that implied there’s a direct relationship (e.g. higher pressure means more fps so if the components generate more pressure irrespective of what they are then you’ll see a difference in velocity/fps if the 28g is constant)

 

from what I can find the velocity can be affected by changing powder primer wad crimp clearly and Folkestone Engineering shows some examples of over crimping causing increased pressure but can’t find anything that answers the above question 

 

D

[Scully]

Of course there is! Never had a real good session followed by a curry? 

[Downforce]

13 minutes ago, Scully said:

Of course there is! Never had a real good session followed by a curry? 

I have

But 5x Murphys (no Curry) and

5x Carlsbergs + Vindaloo can give same results 

so I guess I’m asking about the bit I don’t know much about which is powders that burn faster and slower 

the question would be something like if I had the constant 28g load and constant 1350fps could I generate completely different pressure by using different powder or will the different pressure always be reflected in the velocity 

D

PS don’t drink Murphys any more for precisely that reason 

[Scully]

18 minutes ago, Downforce said:

I have

But 5x Murphys (no Curry) and

5x Carlsbergs + Vindaloo can give same results 

so I guess I’m asking about the bit I don’t know much about which is powders that burn faster and slower 

the question would be something like if I had the constant 28g load and constant 1350fps could I generate completely different pressure by using different powder or will the different pressure always be reflected in the velocity 

D

PS don’t drink Murphys any more for precisely that reason 

😀 I’m sorry, I genuinely don’t know! I was just trying to be funny!
It’s a long time since I reloaded ( and that was for handguns ) and nowadays I don’t concern myself with such  things. However, others do, so I’m sure you’ll get a definitive answer before the days out. 
At a guess I would say yes, there is a direct correlation of one with the other, for the simple reason that if you decrease pressure, velocity is decreased also.
As I recall, Vhit’ 110 I used for .357 mag’ burned faster than the old red ‘Bullseye’ powder I put in my old .45 ACP. Like I said, it’s been a while so I could be completely wrong.  
Doesn’t a faster burning powder increase pressure? 

[Downforce]

It’s a long time since I drank Murphys 

but I can tell you a good story over a beer

D

[Smudger687]

Force/area = Pressure

If area is fixed (eg the surface area of one side of a nitro card), and pressure increases, then force must also increase, which results in greater acceleration for a given mass.

In practical terms, higher pressure will usually lead to greater velocities provided that all other components of the cartridge (barring the powder charge) are kept constant. This isn't especially helpful, however, as it's pretty obvious that increasing a load's powder charge is likely to increase pressure and velocities. 

I think what confuses people is that pressure in load data is concerned only with safety, and refers to maximum chamber pressure. What actually matters in terms of velocity is the area under the pressure curve.

Edited December 20, 2021 by Smudger687

[enfieldspares]

Quote

Hi

im just trying to figure out if I have a factory load of a given weight (say 28g) which is quoted at X psi/bar and Y fps

 

does that mean my 28g home load also measured at Y fps will be the same pressure as the factory load?

 

No, no, no! It does not! If as an extreme you loaded two identical cartridges to have a muzzle velocity of 1400 fps with one ounce of lead and one used Bullseye and the other Blue Dot the likelihood is you'd blow you gun up..literally...with the Bullseye load. Powders burn at various speeds. Fast powders and slow powders and in between. The faster the powder then usually it will give a higher pressure to fire something of X weight at Y velocity than will a slower powder to fire the same something of X weight at the same Y velocity. Please do not judge if a load is safe by whether it projects its payload at the same velocity at the muzzle as does factory ammunition with the same payload weight. IT IS AN UNSAFE METHOD!!!

[Downforce]

13 minutes ago, enfieldspares said:

No, no, no! It does not! If as an extreme you loaded two identical cartridges to have a muzzle velocity of 1400 fps with one ounce of lead and one used Bullseye and the other Blue Dot the likelihood is you'd blow you gun up..literally...with the Bullseye load. Powders burn at various speeds. Fast powders and slow powders and in between. The faster the powder then usually it will give a higher pressure to fire something of X weight at Y velocity than will a slower powder to fire the same something of X weight at the same Y velocity. Please do not judge if a load is safe by whether it projects its payload at the same velocity at the muzzle as does factory ammunition with the same payload weight. IT IS AN UNSAFE METHOD!!!

Thanks that’s exactly what wanted to check 

Obviously checking with a chrono is practical but not everyone has access to a barrel with a calibrated transducer to check the pressure 

D

I am aware there are agencies that can do that testing though 

[enfieldspares]

A rough guide...a very rough guide for true brass cases...and ONLY if you approach with extreme care is case head expansion and primer appearance. That is measure the diameter of fired head of a factor case and the after resizing and reloading that case and firing it IN THE EXACT SAME BARREL IT WAS INITIALLY FIRED IN measuring the case head and comparing what it has expanded out to. You can Google images of both these suggestions I am sure.

With primers again as long as you are using the same type of primer as in the factory ammunition (and in truth that's something you may not be able to find out) you can see if the fired primer is nicely still round at its edges or if it is flat square at its edges. And if it has taken on the imprint of the gun's breech face whilst the factory ammunition primer usually does not.

That flat square edge profile and "printing" on the primer of the contour of the machining on breech or cratering back around the firing pin strike mark then usually indicates high pressure. In rifles not necessarily dangerous pressure but IMHO it is something that should never be hoped to be seen in a shotgun primer.

Backed out primers though aren't always a sign of high pressure. They can also indicate very low pressure. But yes they are an unwelcome sign of high pressure too. When fired the primer (and bullet) start to move outwards away form the expanding gases. The bullet will leave the barrel. But the case as it is expanded by the gases also will be thrust backwards and recapture the primer. In a low pressure round the case won't have given enough force or thrust to the case to achieve that.

So how can the same observed thing be caused by two very different occurrences? The same as a wet pavement outside your house in Winter. It may be wet not because it has rained as you may at first think. It may be wet because it has snowed and the snow has melted!

 

Edited December 20, 2021 by enfieldspares

[Stonepark]

5 hours ago, Downforce said:

Hi

im just trying to figure out if I have a factory load of a given weight (say 28g) which is quoted at X psi/bar and Y fps

 

does that mean my 28g home load also measured at Y fps will be the same pressure as the factory load?

 

probably over simplifying with different wads crimps primers and powder but I read an article from a US professor that implied there’s a direct relationship (e.g. higher pressure means more fps so if the components generate more pressure irrespective of what they are then you’ll see a difference in velocity/fps if the 28g is constant)

 

from what I can find the velocity can be affected by changing powder primer wad crimp clearly and Folkestone Engineering shows some examples of over crimping causing increased pressure but can’t find anything that answers the above question 

 

D

If you are using exact same components, then yes, you can duplicate a factory load including FPS and pressure.

If you change anything pressure and therefore FPS will vary depending on the importance of the component you changed  with rough values below:-

 

Primer +- 1000psi

Case +-200psi except Win Hull +1000psi

Powder +-5000psi

Wad +- 1000psi

Load +-5000psi

Crimp +- 2000psi

 

Hence the need for testing each loading.

 

[Scully]

5 hours ago, Downforce said:

It’s a long time since I drank Murphys 

but I can tell you a good story over a beer

D

😀👍

[Vince Green]

On 20/12/2021 at 17:24, enfieldspares said:

No, no, no! It does not! If as an extreme you loaded two identical cartridges to have a muzzle velocity of 1400 fps with one ounce of lead and one used Bullseye and the other Blue Dot the likelihood is you'd blow you gun up..literally...with the Bullseye load. Powders burn at various speeds. Fast powders and slow powders and in between. The faster the powder then usually it will give a higher pressure to fire something of X weight at Y velocity than will a slower powder to fire the same something of X weight at the same Y velocity. Please do not judge if a load is safe by whether it projects its payload at the same velocity at the muzzle as does factory ammunition with the same payload weight. IT IS AN UNSAFE METHOD!!!

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