Pressure/Velocity Question

Sportdog · April 22, 2009, 11:38 AM

I have a hypothetical question to any ballistic "experts" here on TFL. Rifle "A" shoots a 150 grain bullet @2900fps, 50,000PSI chamber pressure with 54 grains of XYZ powder. Rifle "B" shoots a 150 grain bullet with a chamber pressure of 50,000PSI with 52 grains of XYZ powder. For the sake of this question let's assume all other factors are equal. Will the velocity also be 2900fps? Maybe this is a dumb question but I thought I'd throw it out there anyway. Any thoughts? (Please, no I think you are a dumb a$$ responses please! LOL)

kraigwy · April 22, 2009, 12:19 PM

Not necessarly. Presure dosnt really relate to velocity.

Lets assume, we use Band A, a fast burning powder to get X bullet to go 2500 FPS.

Now we are gonna use Brand B, a slower powder to get the same velocity. The round using Brand A, will normally produce lower presures.

An example, When I first got my Ruger #1, in 204, I chose as a starting point a Accuracy Load from one manual. Using their recommended loading in my gun, I got pressure signs and a velocity of about 3800 fps. I changed nothing but the primers and got 4300 fps, with no pressure signs. Less pressure in this case gave me more velocity.

There is no book answer, every gun, is differant, and every change will give differant results.

I'm not an accuracy fan. I work first on accuracy, keeping pressure low, then check the velocity. Doing that, I can use the sight or scope knobs to make any adjustments I need.

GeauxTide · April 22, 2009, 12:43 PM

What other factors? Same cartridge? Same bullet? Same powder? Different rifles?

kraigwy · April 22, 2009, 01:34 PM

[quote}What other factors? Same cartridge? Same bullet? Same powder? Different rifles?[/quote]

All of the above and then some.

Let me throw in another example: In high power we are taught that (depending on loading) a 10-15 degree in temp will change the impact one minute. OK. We can buy that. But now take this into account. We all know the more we fire a rifle the hotter the barrel an chamber get. But how much? Whats the temp. change between the 1st shot and the 5th shot. There is no practical way to tell. But you can see it can (and does) change the pressure of the cart. How much we don't know.

So you are gonna get differant pressure readings. You may or may not get differant velocity readings.

The real problem, I think, is we are over complicating things. Does these minor changes make that much differance.

Another example of what I;m talking about is using BC in bal Cal. programs. There are to major ones, G1 vs G7 (I'm not even gonna get into a discussion of this). But if you put the two in a Bal Program, you'll find the end result is gonna be differant. How much. I've seen it done and the differance between the G1 & G7 at 1000 yards was .6 inches. A half inch at 1000 yards is way better then I can hold. You'll get more varience in that using the same gun and same lot of ammunition.

We can play with slide rules, calculators and computers all day long, its great fun on a cold winter day when you cant shoot, but in reality I believe one would be farther ahead on the range putting ammo down range putting effort into developing your marksmanship fundlementals rather then worrying about something we have little control in.

JMHO

GeauxTide · April 22, 2009, 02:02 PM

My 40 years experience over the chrony is the smaller the Standard Deviation, the smaller the group. All that other stuff complicates the simple fact that the bullet that the barrel likes the best over the charge that deviates the least will be the most accurate.

HiBC · April 22, 2009, 02:35 PM

Maybe I understand what you are trying to ask.

I think it was,all other things being equal,if 52 gr powder achieved the same pressure in one rifle as 54 achieved in another rifle,would the velocities be equal?

I don't know,for sure.

Problem is,all other things will not be equal,even if they are both Rem 700's of the same model and bbl length,consecutive serial numbers.

The same characteristics that let "CSI" Miami ID the bullet from a bbl tell us each rifle is unique.

And,lot to lot variation in powder,primers,cases,will result in some variation .

What may be useful information,if you find a sweet spot load and velocity and then you run out of Re-19 and get a new 8 lb jug,your chronograph may be as useful as your powder scale in recreating that load,as the new lot of powder may require a slightly different charge weight

brickeyee · April 22, 2009, 03:06 PM

The peak pressure is not what results in the bullets velocity.
The velocity is the pressure applied over the barrel dwell time.

The pressure curve is far from anything linear, so the calculation is not a simple thing obtained by multiplying two numbers.

Peak pressure IS what causes failures in firearms.

SL1 · April 23, 2009, 07:48 AM

+1 for Brickeyee's response.

Saying that "all other things are equal" doesn't work without specifying WHAT things are being considered and what are being ignored. If EVERYTHING was absolutely equal, then the velocities and pressures would be equal with identical charges. Because Sportdog specified different charges giving the same peak pressure, then SOMETHNG ELSE must be different. And, there are a LOT of possibilities.

Peak pressure does not tell you how much pressure acted on the bullet over its entire trip down the bore. If different charge weights are giving the same peak pressure in the same cartridge, then they are almost certainly producing pressure vs bullet travel curves that are different. That means different velocities, maybe a little different and maybe a lot different. It depends on what causes the difference in the chartge weights needed to produce the same peak pressure.

Two hypothetical examples:

1. The difference is due SOLELY to a difference in chamber volume, with the first rifle having a significantly larger chamber than the second. In that case, the expansion ratio ( gas volume with bullet seated vs gas volume as bullet exits the bore) would be smaller for the first gun, and the velocity would calculate out as higher, because the pressure acting on the bullet's base is ALWAYS higher in the first gun, excedpt riht at the peak, when they are equal.

2. The difference is due SOLELY to the "start pressure" needed to get bullet moving and start reducing pressure in the case. This changes the shape of the pressure curve, making the peak earlier and affecting how the powder burns as a function of time. The pressue curves MUST cross between the two cases. It is hard to predict how the integral of one will compare to the integral of the other; it will depend on the powder and other things. So it is hard to say which gun will shoot faster.

There are MANY other hypothetical changes in single parameters that will give the result Sportdog proposed. But, any two real guns typically have more than one parameter different between them. So, it is hard to draw a useful conclusion about velocity in one gun by comparing charge wieghts needed to get the same peak pressure in it and another gun. With really big differences, such as comparing a .308 to a .30-'06, clearly case volume accounts for the charge weight differnces needed to achieve the same peak pressure, and we know that dominates the internal ballistics and will always produce a higher velocity from the bigger case. And, we know that the Weatherby rifles with long free-bored throats give higher velocities with larger powder charges at the same peak pressure when compared to other rifles without free-bore shooting the same cartridges. Again, one parameter is so much different between two guns that it clearly dominates the situation.

But, you really cannot assume that smaller differences you measure in supposely identical guns will give you predictable differences in velocity or pressure. There may be other factors at play that you are NOT measuring that will produce larger differences than the things you are measuring. That is why you really can't predict peak pressure very well with a chronograph, although there are formulas and programs for doing that.

SL1

wncchester · April 23, 2009, 08:52 AM

Brick and SL know what they are talking about. Peak pressure means little so far as velocity goes. It's the time/pressure curve that makes speed and thats what we change when we switch powders, all else being the same.

Sevens · April 23, 2009, 09:03 AM

Quote:

Rifle "A" shoots a 150 grain bullet @2900fps, 50,000PSI chamber pressure with 54 grains of XYZ powder. Rifle "B" shoots a 150 grain bullet with a chamber pressure of 50,000PSI with 52 grains of XYZ powder. For the sake of this question let's assume all other factors are equal. Will the velocity also be 2900fps?

I think you are getting the technical answers that you seek, but your question brings up a question: My question is, how did you come up with the question?

Because the typical reloader simply cannot truly measure chamber pressure. It takes a lab and test equipment to do this sort of thing.

If your question was something like: "When I put 52gr of powder in, I get XXXX velocity, and when I put 54gr of powder I get XXXX velocity, why?" then that would be something that could be answered in theory. But it takes extensive equipment to get hard data for an answer.

We can measure velocity with inexpensive and accurate equipment. But pressure is only something we can simulate with software or take as published test data.

If you are comparing pressure from published test data then you have a real problem messing up the entire question-- because published test data is not likely to be comparable. Way too many variables. Way too many.

Still, it's an interesting question.

Sportdog · April 24, 2009, 01:37 AM

It was just a hypothetical question that crossed my mind. I guess that I already somewhat knew the answer because of the different published data in the different reloading manuals that I have but never really felt that I had a lot of basis for it. Thanks for the explaination. The whole science of powder/primer/bullet/case/chamber size/throating/barrel rifle twists/barrel crowns and air temperature/humidity is quite complex and interests me. Thanks for those who were willing to take the time and pass on some information. Much appreciated!

April 22, 2009, 12:19 PM	#2
kraigwy Senior Member Join Date: June 16, 2008 Location: Wyoming Posts: 11,061	Not necessarly. Presure dosnt really relate to velocity. Lets assume, we use Band A, a fast burning powder to get X bullet to go 2500 FPS. Now we are gonna use Brand B, a slower powder to get the same velocity. The round using Brand A, will normally produce lower presures. An example, When I first got my Ruger #1, in 204, I chose as a starting point a Accuracy Load from one manual. Using their recommended loading in my gun, I got pressure signs and a velocity of about 3800 fps. I changed nothing but the primers and got 4300 fps, with no pressure signs. Less pressure in this case gave me more velocity. There is no book answer, every gun, is differant, and every change will give differant results. I'm not an accuracy fan. I work first on accuracy, keeping pressure low, then check the velocity. Doing that, I can use the sight or scope knobs to make any adjustments I need. __________________ Kraig Stuart CPT USAR Ret USAMU Sniper School Distinguished Rifle Badge 1071

April 22, 2009, 01:34 PM	#4
kraigwy Senior Member Join Date: June 16, 2008 Location: Wyoming Posts: 11,061	[quote}What other factors? Same cartridge? Same bullet? Same powder? Different rifles?[/quote] All of the above and then some. Let me throw in another example: In high power we are taught that (depending on loading) a 10-15 degree in temp will change the impact one minute. OK. We can buy that. But now take this into account. We all know the more we fire a rifle the hotter the barrel an chamber get. But how much? Whats the temp. change between the 1st shot and the 5th shot. There is no practical way to tell. But you can see it can (and does) change the pressure of the cart. How much we don't know. So you are gonna get differant pressure readings. You may or may not get differant velocity readings. The real problem, I think, is we are over complicating things. Does these minor changes make that much differance. Another example of what I;m talking about is using BC in bal Cal. programs. There are to major ones, G1 vs G7 (I'm not even gonna get into a discussion of this). But if you put the two in a Bal Program, you'll find the end result is gonna be differant. How much. I've seen it done and the differance between the G1 & G7 at 1000 yards was .6 inches. A half inch at 1000 yards is way better then I can hold. You'll get more varience in that using the same gun and same lot of ammunition. We can play with slide rules, calculators and computers all day long, its great fun on a cold winter day when you cant shoot, but in reality I believe one would be farther ahead on the range putting ammo down range putting effort into developing your marksmanship fundlementals rather then worrying about something we have little control in. JMHO __________________ Kraig Stuart CPT USAR Ret USAMU Sniper School Distinguished Rifle Badge 1071

April 24, 2009, 01:37 AM	#11
Sportdog Junior member Join Date: February 28, 2006 Location: Southwestern Michigan Posts: 369	Sevens It was just a hypothetical question that crossed my mind. I guess that I already somewhat knew the answer because of the different published data in the different reloading manuals that I have but never really felt that I had a lot of basis for it. Thanks for the explaination. The whole science of powder/primer/bullet/case/chamber size/throating/barrel rifle twists/barrel crowns and air temperature/humidity is quite complex and interests me. Thanks for those who were willing to take the time and pass on some information. Much appreciated!

April 22, 2009, 11:38 AM	#1
Sportdog Junior member Join Date: February 28, 2006 Location: Southwestern Michigan Posts: 369	Pressure/Velocity Question I have a hypothetical question to any ballistic "experts" here on TFL. Rifle "A" shoots a 150 grain bullet @2900fps, 50,000PSI chamber pressure with 54 grains of XYZ powder. Rifle "B" shoots a 150 grain bullet with a chamber pressure of 50,000PSI with 52 grains of XYZ powder. For the sake of this question let's assume all other factors are equal. Will the velocity also be 2900fps? Maybe this is a dumb question but I thought I'd throw it out there anyway. Any thoughts? (Please, no I think you are a dumb a$$ responses please! LOL)

April 22, 2009, 12:43 PM	#3
GeauxTide Senior Member Join Date: April 20, 2009 Location: Helena, AL Posts: 4,424	What other factors? Same cartridge? Same bullet? Same powder? Different rifles?

April 22, 2009, 02:02 PM	#5
GeauxTide Senior Member Join Date: April 20, 2009 Location: Helena, AL Posts: 4,424	My 40 years experience over the chrony is the smaller the Standard Deviation, the smaller the group. All that other stuff complicates the simple fact that the bullet that the barrel likes the best over the charge that deviates the least will be the most accurate.

April 22, 2009, 02:35 PM	#6
HiBC Senior Member Join Date: November 13, 2006 Posts: 8,288	Maybe I understand what you are trying to ask. I think it was,all other things being equal,if 52 gr powder achieved the same pressure in one rifle as 54 achieved in another rifle,would the velocities be equal? I don't know,for sure. Problem is,all other things will not be equal,even if they are both Rem 700's of the same model and bbl length,consecutive serial numbers. The same characteristics that let "CSI" Miami ID the bullet from a bbl tell us each rifle is unique. And,lot to lot variation in powder,primers,cases,will result in some variation . What may be useful information,if you find a sweet spot load and velocity and then you run out of Re-19 and get a new 8 lb jug,your chronograph may be as useful as your powder scale in recreating that load,as the new lot of powder may require a slightly different charge weight

April 22, 2009, 03:06 PM	#7
brickeyee Senior Member Join Date: December 29, 2004 Posts: 3,351	The peak pressure is not what results in the bullets velocity. The velocity is the pressure applied over the barrel dwell time. The pressure curve is far from anything linear, so the calculation is not a simple thing obtained by multiplying two numbers. Peak pressure IS what causes failures in firearms.

April 23, 2009, 07:48 AM	#8
SL1 Senior Member Join Date: November 8, 2007 Posts: 2,001	+1 for Brickeyee's response. Saying that "all other things are equal" doesn't work without specifying WHAT things are being considered and what are being ignored. If EVERYTHING was absolutely equal, then the velocities and pressures would be equal with identical charges. Because Sportdog specified different charges giving the same peak pressure, then SOMETHNG ELSE must be different. And, there are a LOT of possibilities. Peak pressure does not tell you how much pressure acted on the bullet over its entire trip down the bore. If different charge weights are giving the same peak pressure in the same cartridge, then they are almost certainly producing pressure vs bullet travel curves that are different. That means different velocities, maybe a little different and maybe a lot different. It depends on what causes the difference in the chartge weights needed to produce the same peak pressure. Two hypothetical examples: 1. The difference is due SOLELY to a difference in chamber volume, with the first rifle having a significantly larger chamber than the second. In that case, the expansion ratio ( gas volume with bullet seated vs gas volume as bullet exits the bore) would be smaller for the first gun, and the velocity would calculate out as higher, because the pressure acting on the bullet's base is ALWAYS higher in the first gun, excedpt riht at the peak, when they are equal. 2. The difference is due SOLELY to the "start pressure" needed to get bullet moving and start reducing pressure in the case. This changes the shape of the pressure curve, making the peak earlier and affecting how the powder burns as a function of time. The pressue curves MUST cross between the two cases. It is hard to predict how the integral of one will compare to the integral of the other; it will depend on the powder and other things. So it is hard to say which gun will shoot faster. There are MANY other hypothetical changes in single parameters that will give the result Sportdog proposed. But, any two real guns typically have more than one parameter different between them. So, it is hard to draw a useful conclusion about velocity in one gun by comparing charge wieghts needed to get the same peak pressure in it and another gun. With really big differences, such as comparing a .308 to a .30-'06, clearly case volume accounts for the charge weight differnces needed to achieve the same peak pressure, and we know that dominates the internal ballistics and will always produce a higher velocity from the bigger case. And, we know that the Weatherby rifles with long free-bored throats give higher velocities with larger powder charges at the same peak pressure when compared to other rifles without free-bore shooting the same cartridges. Again, one parameter is so much different between two guns that it clearly dominates the situation. But, you really cannot assume that smaller differences you measure in supposely identical guns will give you predictable differences in velocity or pressure. There may be other factors at play that you are NOT measuring that will produce larger differences than the things you are measuring. That is why you really can't predict peak pressure very well with a chronograph, although there are formulas and programs for doing that. SL1

April 23, 2009, 08:52 AM	#9
wncchester Senior Member Join Date: December 1, 2002 Posts: 2,832	Brick and SL know what they are talking about. Peak pressure means little so far as velocity goes. It's the time/pressure curve that makes speed and thats what we change when we switch powders, all else being the same.