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Post by Jon on Dec 19, 2010 9:52:34 GMT -5
Guys just my 2cents if I remember correctly Rb's 40000 limit was for sabot loads as much beyond this was where the sabot could fail. I think he used 45000 for bare bullets. I may be wrong if so someone correct me. Jon
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Post by Richard on Dec 19, 2010 16:16:48 GMT -5
TG.........they seem to mention the word "chrome moly" but not stainless steel? Could it be they are comparable in stretch characteristics? Richard
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Post by Jon on Dec 19, 2010 18:25:56 GMT -5
Richard. Good question you beat me to it. Jon
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Post by chuck41 on Dec 19, 2010 19:29:57 GMT -5
ET, The pressure trace machine estimates the velocity based on the pressures it measures along with the length of barrel, the diameter, and the weight of the bullet. It has to use a combination of several mathematical formulas to calculate that. F=MA is the first of them and that means (F) force (pressure applied to base of bullet) = (M) mass (in our case measured in grains of weight) times (A) Acceleration. Another is V = AT (V) Velocity = (A) Acceleration x (T) time (which is directly measured by the unit). The force is equal to the pressure calculated by the trace unit integrated over the base area of the bullet and varies as you see it plotted on the trace. It is only by calculating these things that the system can give you a muzzle velocity. If the muzzle velocity is correct as measured from your crony that tells you that the measured pressures are also correct.
There is one other variable that is not mentioned here and that is internal friction and that is estimated in the machine based on what is expected from a similar cartridge gun. This also assumes a certain land and grove configuration which is likely different in whatever gun you might use, but is likely not enough of a difference to matter much.
The manner in which pressure is calculated is immaterial to these formulas. It would be the same if there were a pressure sensor at a point tapped off on the barrel to a direct pressure transducer. If the velocity it is estimating is the same as the crony shows it means the variables the trace machine is using matches reality. If it does not match that tells you they do not. A strain gauge can only provide an input that "guesses" what the pressures must be to make a piece of steel of a certain metallurgy and certain configurations as to thickness and configuration flex enough to change the electrical resistance of the strain gauge. Stainless will be different from chrom-molly and different alloys of chrom-molly will vary as well according to the relative toughness of the alloy used. That's why they tell you to use a known pressure producing factory round to calibrate the system. Every gun is going to be a bit different even if the strain gauges used are very consistent. Remember, the trace unit does not MEASURE PRESSURE. It calculates what it must be based on the amount of flexure of the barrel.
The mass of the bullets (of the same weight) are a constant and are directly measurable. Time is directly measured. The pressures and the resulting acceleration are only an estimate. The velocity you can actually measure with your chrony. If the velocity calculated by the trace unit matches the one measured by the chrony the true pressure must also be very close to what is calculated since it is that force acting on the base of the bullet that produces the velocity.
Does it tell you precisely what the internal pressure really is? Of course not. It is just very much better that what you have by simply estimating all those factors you entered into the machine and hoping for the best, since you do not know the precise metallurgy and similar variables that enter into the equations. You can calculate one unknown when you can actually measure the others. Unfortunately here we have two variables we can not directly measure. Acceleration and Pressure (Force).
Previously Edge and Richard had a couple traces with 62gr 4198 showing very large pressure differentials (9K psi) with bullets that were only 5 grains different, and the lighter bullet generating the higher pressure. Common sense tells you that at least one of those is quite a bit off. This will tell you which one is the closer to reality and likely even how much closer as well.
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Post by edge on Dec 19, 2010 20:29:52 GMT -5
Are you sure that the PT calculates velocity?
Everything that I have read is you must input the velocity, but I may have missed it.
edge.
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Post by ET on Dec 19, 2010 21:27:14 GMT -5
Chuck41
I partly agree with what you are saying but I don’t see the pressure trace unit using the calculated peak pressure to assist with deriving at muzzle velocity. The strain gauge can only be used to establish the pressure at its location point and not throughout the barrel length. Here I see a time distance relationship being somehow employed to derive muzzle velocity. No argument about the formula of F=MA. But is the Force constant behind the mass going through the barrel? If the force is not constant then the rate of acceleration isn’t constant either. Without constant values put into this formula how can we derive an answer?
Okay do we agree that by using a different powder say a slower burning powder with a given mass that we can keep the peak pressure from not exceeding the original peak pressure from a faster burning powder yet with the extended burn increase muzzle velocity of the projectile? If you agree then the graph should show a shorter barrel time relationship and not a major difference in peak pressure. Again it is my belief that the barrel time relationship is being employed to calculate the muzzle velocity.
Chuch41 great discussion for looking at different view points.
Edge
Check page 1 of this thread and the attachment I included.
Ed
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Post by chuck41 on Dec 20, 2010 0:52:38 GMT -5
Chuck41 I partly agree with what you are saying but I don’t see the pressure trace unit using the calculated peak pressure to assist with deriving at muzzle velocity. The strain gauge can only be used to establish the pressure at its location point and not throughout the barrel length. Here I see a time distance relationship being somehow employed to derive muzzle velocity. No argument about the formula of F=MA. But is the Force constant behind the mass going through the barrel? If the force is not constant then the rate of acceleration isn’t constant either. Without constant values put into this formula how can we derive an answer? Chuch41 great discussion for looking at different view points. Ed The force and acceleration are not constants and you cannot calculate a muzzle velocity from peak pressure. A very fast pistol powder that produces the same peak pressure as a very slow burning rifle powder will create a much slower muzzle velocity. However, the trace unit has all the info it needs to calculate that from the instantaneous pressures the unit calculates and displays on your trace. The computer (my aren't they amazing) calculates a value using integral calculus from time t=0 to time t=x (where "x" represents the time the bullet actually leaves the barrel). V = A*T and V = the sum of (A1*T1 + A2*T2 + A3*T3 . . . . . . . + Ax*Tx) It is like dividing the time from ignition until the bullet exits into ten thousand tiny little pieces and calculating the increase in velocity of the bullet for each little piece of time based on the pressure at that instant of time then adding them all together to get the muzzle velocity. That is the way that type of calculation is normally accomplished. The pressure trace that you see is not the pressure calculated at a given point on the barrel, it represents the pressure experienced everywhere in the barrel behind the obstruction caused by the bullet and how that pressure changes over time. That's why they are so strict on where the gauge has to be located. If it is located halfway down the barrel it is pretty much worthless as it won't give much data about what happened before the bullet reaches that location. What is displayed is based on the strain experienced at the location where the gauge is glued on. It is for all practical purposes the same pressure seen acting at the base of the bullet at any instant of time until it actually leaves the barrel. Thus the peak pressure experienced in the barrel is experienced near the breech end of the barrel and the muzzle of the barrel will likely never experience more that a few thousand psi unless exceedingly slow powders are used. The pressure normally will have decreased by time the bullet passes that point so gun barrels don't have to be so thick there, at least not for safety. If your pressure trace can give you an indicated muzzle velocity, that is the only method it has to calculate it. The more accurately that calculated velocity represents a chrony measured muzzle velocity, the more accurate the pressure it uses to make that calculation must be as well. A slower burning powder will achieve it's max pressure later and with the same quantity of powder the max pressure will be lower, but will be acting on the bullet over a longer time. If you then put in more powder so the max pressure is the same, the velocity will be significantly higher since it the slower burn means it will be acting on the bullet for a much longer time. Then if you get real smart and use a fancy duplex you get an initial quick rise in pressure from the fast burning booster powder followed by a much longer sustain near peak pressure contributed by the slower burning main powder to get possibly even much higher velocity. With a "perfect load" (never happen) the pressure trace would go almost straight up to its maximum safe value then maintain that value until the bullet approaches the muzzle where it would quickly drop to near nothing as the bullet exits. That way you would have maximum bullet velocity with minimum muzzle flash and maximum efficient use of the fully burned powder.
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Post by spaniel on Dec 20, 2010 7:53:41 GMT -5
Worthy of quoting as there seems to be some confusion on this point.
With an expanding cylindrical chamber behind the bullet, pressure equalizes instantaneously behind the bullet; for the bullet to move forward an equal and opposite force must be transferred backwards against the breech so by definition the pressure is the same throughout the barrel. Barrels can be thinner at the muzzle because by then pressure has dropped everywhere in the barrel.
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Post by edge on Dec 20, 2010 8:15:45 GMT -5
SNIP Edge Check page 1 of this thread and the attachment I included. Ed ET, Your attachment was cut off too early IMO. The paragraph a bit further down in AREA explains that this information is calculated using the velocity information that you enter for each shot. While it probably could calculate velocities the information entered into the unit is not nearly enough for those calculations...IMO edge.
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Post by ET on Dec 20, 2010 8:25:22 GMT -5
Chuck41
You got me on the calculus because I never learned that mathematical form. If the unit is actually using measured points on the resulting graph then the graph characteristic would influence the velocity calculation. Then yes I would have to agree that a possible form of calibration could be made with the use of a chrony for verifying accuracy through resulting velocity comparison.
Okay time to go back to the drawing board and examine a new picture with an open mind.
Ed
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Post by ET on Dec 20, 2010 9:02:17 GMT -5
Edge Sorry I should have included this scanned picture from the manual. If you look at the bottom right hand corner there are 2-colored boxes. The Blue box deals with parameters required for velocity calculation. This is from the 4.5.0 version that I now have installed. Ed
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Post by edge on Dec 20, 2010 13:04:15 GMT -5
I heard back from Jim @ RSI
edge.
Mark; I think they are confusing the complete RSI Shooting Lab system with software, M2 Chronograph and PressureTrace. You are correct. You need a chronograph to get velocities and PT does not provide velocities.
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Post by ET on Dec 20, 2010 13:53:36 GMT -5
Now that is getting pretty bad and misleading to mention parameters for velocity calculation and then say you need a chronograph for velocities. Wonder how much more misleading info is in the manual that is going to be found?
Okay I got lead down the garden path on this one. At least I don’t have to wait till spring to find out about it.
Ed
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Post by edge on Dec 20, 2010 14:04:44 GMT -5
Actually I did not find it misleading since it said that you needed to input the velocities.
The barrel length and velocities are to come up with the OBT times and really is just an added feature...if you subscribe to the harmonic node theories of the author.
They have no bearing on the pressure curves.
edge.
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Post by Jon on Dec 20, 2010 16:49:03 GMT -5
I do know that when you buy the pt they have what they call a complete system it includes the pt some soft ware and a crony. So I get the impression they feel a crony is needed to get all the answers? Jon
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Post by Jon on Dec 20, 2010 16:51:25 GMT -5
Sorry I didn't read back far enough Edge has already been given the answer. Jon
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Post by chuck41 on Dec 20, 2010 17:43:27 GMT -5
Tis a shame. From Jon's post it sounded like they has some super software in that thing. Back to plan "B".
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Post by Richard on Dec 20, 2010 21:10:54 GMT -5
And, you will also note that there is no place to input bullet weight aside from that "verification load". As Chuck was mentioning, you needed bullet weight for velocity. Even bullet diameter is not necessary for pressure calculations. I am planning on trying 60 gr. of H-4198 tomorrow and shooting 200, 230 and 250 gr. XTP's (sabot less) all with the same charge. Hopefully we can see what the difference in pressure is as we increase the weight with the same powder charge. I will also adjust the trigger sensitivity up to hopefully eliminate or minimize those little squiggly lines at the bottom. Richard
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Post by edge on Dec 21, 2010 10:36:32 GMT -5
FYI, if I put that load in to QL I get the following estimates @ 100.1% loading capacity : 200 = 2642 fps @ 39167 psi 230 = 2581 fps @ 43210 psi 250 = 2540 fps @ 45783 psi It will be interesting to see your velocities and pressures recorded. edge. PS those velocities are for a bullet travel of 21 5/8 I don't know the usable length of your barrel, but peak pressure is still the same anyway PPS I forgot to compensate for temperature too If I tell it that the temperature is 50 F, then the 250 grain bullet now becomes 2514 fps and 44385 psi.
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