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Thread: pressure=speed?

  1. #1

    Question pressure=speed?

    More powder, hotter primers, longer and harder bearing surfaces, increase or decrease in air tempature,etc., increases pressures, but my question is...does increased pressure ALWAYS result in increased speed? Are there any scenarios where it doesn't?
    If you like getting kicked by a mule...then you'll "love" shooting my .458.

  2. #2

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    Sure. Max out pressure in a handgun with Bullseye, and you'll still have only mediocre velocity. Switch to a slow burning powder and it's easy to beat that velocity at much lower pressure.

    That's an extreme example, but you can do the same thing with 3031 and RL-22 in a rifle.

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    Does increased pressure always result in increased speed? No.

    Let's assume here you mean in the same gun. A particular load that produces 54,000 psi and 2614 fps in rifle A may produce 62,000psi and 2545 fps in rifle B.

    In the same gun:

    You can develop a load that reaches a max mean pressure of 60,000 psi and a velocity of 2750 fps with one particular powder and another load, with a different powder, that develops 65,000psi and 2700 fps. Or something along those lines.

    Are there any scenarios where it doesn't? Yes
    Staying with all the same components:

    When developing a load and working up one grain at a time and chronographing each shot, I have seen velocity increase in rather predictable steps. Such as 40-50 fps per grain as pressure goes up. (watching with a strain gage and an Oehler M43 system) Then as we reach near max pressure the steps of velocity increase get smaller, 25-30 fps per grain, until added powder does not increase velocity but pressure still goes up. This is what I call the knee of the curve and indicates a max load. (Actually the max load is the last full step of velocity at 1 grain increase of powder.)

    If you use one powder in the same rifle and all the same components from the same lot and, as you add powder, velocity fails to increase, you are already in trouble. The pressure can always be expected to increase as powder is added or any increase in the rate of confinement of the load. (heavier bullet, tighter crimp, bullet jammed into the rifling, etc.)

    Also it is possible when using the wrong burning rate of powder to add powder and not see any increase in velocity as the powder is not burning (given the expansion ratio vs burn rate) and is just being blown out the barrel. Such as very slow powder in large bore straight sided cases. (I.E. 4831 and 45-70)

    There is a point in the barrel where we say the powder is "all burnt" This doesn't mean it is all burned but all that is going to be burned is burned. Generally that is a few inches down the tube and at the peak pressure for the load. (Expanson ratio vs burn rate is important.)

    I think barrel friction has an enormous effect on pressure vs velocity. I have some very smooth hand lapped barrels that give very high velocity with moderate charges of powder. I think I get more fps per psi with these guns. Of course barrel dimensions plays a part in this as well. I have fired the same load with new unfired brass (30-06) from six different rifles, with the same barrel length, and got velocity variations of as much as 220 fps difference in velocity. The smooth barrel guns were the fastest and when I attached the strain gage, they also had the lowest mean pressure. I believe that is a valid test.

    I've read several times, info by the gun writers that says if you exceed factory velocity you exceed factory pressure. Though that is good advice from a safety stand point, it isn't always true.

    I'm not sure if your questions were in regard to handoading but assume so. I'm curious what gives?
    Is there nothing so sacred on this earth that you aren't willing to kill or die for?



  4. #4

    Smile mainly curiousity.

    We were shooting yesterday...I was adjusting sight for a specific hunting bullet and it was very hot, so I kept my loads on ice, as to have air temp not be a major factor. Just got to thinking about the pressure thing because there are so many factors that make pressures go up, I was wondering if speed increases in all these scenarios. I know that I usually get higher poi on hot days and that a max load at lower temp (say around 45-45 degrees) is a max load on steroids when it's hot (85 +). Most of my hunting is usually at lower temps but I also want safe loads on that occassional hot spring or fall day. Not much will torque your tush more than a stuck case on a hunting trip.Ciao.
    If you like getting kicked by a mule...then you'll "love" shooting my .458.

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    The classic experiment was Hodgdon's .264-378 Weatherby. At the max pressure it still only produced 3450-3500fps. I believe it was run at 65kpsi.

  6. #6

    Question barrel length

    Another thought. Does barrel length have any effect on pressure or once the pressure has spiked, then any excess barrel length is a moot point? It seems to must have some effect to one degree or another (to a point) because lopping off a few inches decreases velocity. With faster burning powders, the shorter the barrel can be, right?
    If you like getting kicked by a mule...then you'll "love" shooting my .458.

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    I love this subject, as you may have seen before.

    I wish I could draw you a curve to represent pressure. (Picture if you will a human leg viewed from the lateral with the knee up and the foot flat on the floor) This would be a rapid rising line to represent pressure on the vertical axis and time on the horizontal. The curve rises rapidly (the thigh) to peak pressure (the knee) of about 60kpsi then decays much more slowly (the lower leg) down to somewhere around 20kpsi (the ankle), then the bullet exits. (Now that I have your attention.)

    This represents the rapid rise of pressure inside a barrel. As the bullet begins to move fast enough that the pressure cannot continue to increase, pressure begins to drop, this is the peak, the knee of the curve. Faster burning powders peak sooner but the difference is small. This horizontal axis of time also equates to the barrel length of the gun and the peak will be only a few inches in front of where the bullet lies before firing. The peak is moved no more than an inch as we vary the burning rate of powders, so it has some effect but not much. Mostly the advantage of faster powders for shorter barrels is lower exit pressure and less noise and flash. Exit pressures will normally be between 10 and 30 kpsi. Faster powders do accelerate the bullet more quickly but peak sooner so must be used in lesser quantity.

    As the pressure curve decays, the bullet is still being accelerated down the barrel. Shortening the barrel of a rifle will exit the bullet sooner and cut short the acceleration time and therefore reduce the muzzle velocity. Pressure will still be at a higher level and that gives us more bang and flash. And as I said, faster powders will be at a lower exit pressure.

    By faster and slower powders, I'm referring to powders appropriate for the caliber. For the 30-06 as an example, 4895, 4064 and RL-15 are faster and 4350, RL-19, 4831 and RL-22 are slower. The difference in exit pressure for a 24" barrel for a "standard" load would be 4064 = 14,000 psi, 4831 = 22,000 psi. The latter will bend the actuating rod on an M1 Garrand rifle as it is designed to operate at 14,500 psi at the port. The shorter tanker rifles have smaller ports to handle the higher pressure as the exit is farther up the leg of the curve.

    I hope you have enjoyed this early morning lesson in anatomy...er, ah..... I..... mean .....internal ballistics.
    Is there nothing so sacred on this earth that you aren't willing to kill or die for?



  8. #8

    Arrow OK Teach

    So can I assume that a bullet will continue to increase in speed in an infinite barrel length until the bullet drag equals or overcomes the internal pressure factor?
    If you like getting kicked by a mule...then you'll "love" shooting my .458.

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    Quote Originally Posted by Maydog View Post
    So can I assume that a bullet will continue to increase in speed in an infinite barrel length until the bullet drag equals or overcomes the internal pressure factor?
    Yes, and the bullet will come to a stop somewhere in the barrel and behind the bullet, pressure will be at some level above atmosphere and below peak.
    Is there nothing so sacred on this earth that you aren't willing to kill or die for?



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    Default simple example

    Quote Originally Posted by Maydog View Post
    More powder, hotter primers, longer and harder bearing surfaces, increase or decrease in air tempature,etc., increases pressures, but my question is...does increased pressure ALWAYS result in increased speed? Are there any scenarios where it doesn't?
    Shoot the exact same load in a 20" chrome moly or stainless barrel then a 20" chrome lined barrel.

    I have always found higher velocity with less pressure from exact same loads in the chrome lined barrel.

    Found this to be true in all military style autoloaders I have tested-AR15 AR10 FNFAL M1A...M9 pistols as well.

    So at the worst you will get higher velocity with the same pressure but most often I think you should get higher velocity with less pressure because of less friction of chrome.

    Oh, for a chrome bolt gun barrel with a chrome chamber that never rusts, higher velocity, less fouling bore and less pressure with better extraction....

    jedi

  11. #11

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    This fascinates me. I guess I always worked off the assumption that pressure=velocity. So I have a question- if there are two loads (one uses more of one powder and the other uses less of another powder) that both drive the same bullet to the same velocity out of the same gun, is there ever a scenario in which the smaller load would produce more pressure?

    If so, does that translate into recoil? Let me rephrase that to parrallel the first question: if there are two loads (one uses more of one powder and the other uses less of another powder) that both drive the same bullet to the same velocity out of the same gun, is there ever a scenario in which the smaller load would produce more recoil?

  12. #12

    Talking probably

    Quote Originally Posted by go_north View Post
    This fascinates me. I guess I always worked off the assumption that pressure=velocity. So I have a question- if there are two loads (one uses more of one powder and the other uses less of another powder) that both drive the same bullet to the same velocity out of the same gun, is there ever a scenario in which the smaller load would produce more pressure?

    If so, does that translate into recoil? Let me rephrase that to parrallel the first question: if there are two loads (one uses more of one powder and the other uses less of another powder) that both drive the same bullet to the same velocity out of the same gun, is there ever a scenario in which the smaller load would produce more recoil?
    I would think so, especially if the smaller or less load was a faster burning rate, hence peaking somewhat sooner. If everything else is the same, and velocity is the same, then the smaller load almost has to be a faster rate...right? Makes sense to me...and then recoil might be sharper,but probably not much noteable difference. OK, I'll go sit in the corner, ciao.
    If you like getting kicked by a mule...then you'll "love" shooting my .458.

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    Pressure = velocity UNTIL you reach a condition of choked flow. Choked flow in a pipe or rifle barrel -occurs when the speed of the gas is equal to the speed of sound at that pressure and temperature. At choked flow increasing the pressure at the case creates very little increase in velocity as the speed of sound does not change significantly.

    Some years ago I calculated the choked flow for hot nitrogen and CO2 the primary components when smokeless powder burns at a pressure of 65,000 psi. The calculated choked flow velocity was around 4000 fps- close to what we see in real life.

    There are many other factors to consider of course but the choked flow appears to be the limiting factor of velocity in a common rifle. Ill have to re-run the calculations when I get time and post the results as well as the gas composition and temperature I used.

    In my daytime job I run into the problem of choking in the sizing of valves and equipment for high pressure gas blow-down from gas wells and processing equipment.

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    Quote Originally Posted by Maydog View Post
    Another thought. Does barrel length have any effect on pressure or once the pressure has spiked, then any excess barrel length is a moot point? It seems to must have some effect to one degree or another (to a point) because lopping off a few inches decreases velocity. With faster burning powders, the shorter the barrel can be, right?
    I didn't really answer this so here goes.
    Does barrel length have any effect on pressure? Yes, but.

    Expansion ratio is the ratio of the volume of the powder chamber to the volume of the powder chamber plus the swept volume of the barrel (the barrel through which the bullet travels). Shortening the barrel changes this expansion ratio and changes the internal dynamics of the expanding gases within this volume. This does not necessarily mean the pressure goes up or down but it does change the what is the most efficient burning rate of powder, which will cause a change in pressure vs time and change the exit pressure of the load. We're splitting a lot of fine hairs here and it doesn't amount to much in pressure, velocity or recoil.
    Is there nothing so sacred on this earth that you aren't willing to kill or die for?



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    Quote Originally Posted by go_north View Post
    This fascinates me. I guess I always worked off the assumption that pressure=velocity. So I have a question- if there are two loads (one uses more of one powder and the other uses less of another powder) that both drive the same bullet to the same velocity out of the same gun, is there ever a scenario in which the smaller load would produce more pressure?

    If so, does that translate into recoil? Let me rephrase that to parrallel the first question: if there are two loads (one uses more of one powder and the other uses less of another powder) that both drive the same bullet to the same velocity out of the same gun, is there ever a scenario in which the smaller load would produce more recoil?
    Generally more powder means more recoil.

    Recoil Impulse is calculated to include the weight of powder and the exit velocity of the hot gases produced by this charge of powder. I use a standard for this exit velocity in my formula below of 4400 fps. This is actually low for many higher pressure cartridges of today and will range from 4400 fps to 5200 fps or more.

    If a lesser charge of powder "A" produced greater exit pressure and gas exit velocity, than a greater charge of powder "B" could produce, then yes the recoil could be greater. We can plug numbers into this formula all day long and come up with various changes in the recoil impulse, etc.

    A difficulty here is our ability to measure this gas exit velocity also the difference in recoil would likely be unnoticable.

    I posted this on this forum some time back and here it is again for your viewing pleasure. You don't actually have to do the math to see what does and doesn't effect recoil. This is my own creation so be gentle with me.



    Recoil is calculated in three aspects.

    Recoil Impulse (RI), Recoil velocity (RV) and Recoil Energy (RE. For best results, in that order.


    RI=(Bw*Bv) + (Pw*4400)/225190 - lbs/sec

    RV=RI*32.17/Gw - ft/sec

    RE=Gw*RV(squared)/64.34 - ft lbs

    Bw=Bullet weight in grains.
    Bv= Bullet velocity in ft/sec.
    Pw=Powder weight in grains
    Gw=Gun weight in pounds

    4400 is the constant which represents the exit velocity of the powder gasses. (a major part of recoil, I call it the Bill Lear effect)

    225190 is the acceleration of gravity (g) 32.17 ft/sec*7000(the number of grains in a pound) This constant gets it back to common units.

    g=32.17 ft/sec

    g2=64.34 ft/sec

    This will provide a close approximation of recoil. We are generally interested in Recoil Energy and Recoil velocity.

    A 375 with a 300 grain bullet and 80 grains of powder which gives 2600 fps velocity in an eight pound rifle will produce about 50 ft lbs of RE and an RV of 20 fps.
    Is there nothing so sacred on this earth that you aren't willing to kill or die for?



  16. #16

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    So- if I'm looking for load info for a new gun, and I find two different loads that use two different powders, but they drive the same bullet to the same velocity, is it safe to assume that the smaller load will recoil less?

    Or is the gas-exit-velocity a likely enough variable that it's NOT safe to assume that?

    Is there any reason to choose the larger load if I get no ballistic advantage?

  17. #17

    Talking maybe

    Quote Originally Posted by go_north View Post
    So- if I'm looking for load info for a new gun, and I find two different loads that use two different powders, but they drive the same bullet to the same velocity, is it safe to assume that the smaller load will recoil less?

    Or is the gas-exit-velocity a likely enough variable that it's NOT safe to assume that?

    Is there any reason to choose the larger load if I get no ballistic advantage?
    In my mind, accuracy in your particular rifle could be a reason, but I doubt felt recoil would be noticeable enough to make the decision on.
    If you like getting kicked by a mule...then you'll "love" shooting my .458.

  18. #18
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    Arrow Tighter groups

    Murphy, you're missing an important pair of parentheses there.
    The formulas work a lot better like this:

    Quote Originally Posted by Murphy View Post
    ...
    RI=((Bw*Bv) + (Pw*4400))/225190 - lbs/sec

    RV=RI*32.17/Gw - ft/sec

    RE=Gw*RV(squared)/64.34 - ft lbs

    Bw=Bullet weight in grains.
    Bv= Bullet velocity in ft/sec.
    Pw=Powder weight in grains
    Gw=Gun weight in pounds

    4400 is the constant which represents the exit velocity of the powder gasses. (a major part of recoil, I call it the Bill Lear effect)

    225190 is the acceleration of gravity (g) 32.17 ft/sec*7000(the number of grains in a pound) This constant gets it back to common units.

    g=32.17 ft/sec

    g2=64.34 ft/sec

    ...

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    Quote Originally Posted by go_north View Post
    So- if I'm looking for load info for a new gun, and I find two different loads that use two different powders, but they drive the same bullet to the same velocity, is it safe to assume that the smaller load will recoil less?
    ...
    Is there any reason to choose the larger load if I get no ballistic advantage?
    Well, Maydog gave my primary answer:
    Quote Originally Posted by Maydog View Post
    In my mind, accuracy in your particular rifle could be a reason...
    Secondarily, if Murphy's numbers are spot-on, exit velocity of the combusted gasses can go up 18% (from 4400-5200). An 18% difference in charge weight yielding the same bullet velocity is unlikely. So in the even more unlikely hypothetical situation that the lighter charge had max gas exit velocity, and the (18%) heavier charge was at the bottom of the range for gas exit velocity, they would 'kick' the same.

    It's also worthwhile, if you're working up a load to be used under field conditions, to try and find a powder that fills the case with minimal extra room for settling or shifting.

    Lacking any way at all to measure gas exit velocities, the one recoil variable you can control in a given rifle and bullet weight is the powder weight. So accuracy first, then matching powder volume to case volume, and finally powder charge, if all else is equal.

    That's a really long-winded way of saying I wouldn't worry about it.

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    Quote Originally Posted by 8x57 Mauser View Post
    Murphy, you're missing an important pair of parentheses there.
    The formulas work a lot better like this:
    Yeah, you're probably right. I just used the vinculum as a sign of grouping but the parents do make it easier to understand.

    It's really just RI=bm*bv+pm*pv but with the pounds vs grains and gravity and mass vs weight and the angle of the sun and.......
    Is there nothing so sacred on this earth that you aren't willing to kill or die for?



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