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 July 31, 2012, 10:08 AM #1 ThisIsMySig Member   Join Date: June 6, 2012 Location: Apache Junction, Arizona Posts: 65 Calculating Velocity The velocity numbers in my manual are stated only for the min and max loads. Does anyone know the formula for figuring velocity for loads in between? I imagine it would be a relatively complex formula. __________________ Sig P250 in 9mm and .45 cal, Sig 1911 .22LR, Heritage Arms 32 H&R Magnum revolver, S&W Governor. Reloading 9mm, 32 magnum, 45 ACP, 45 LC and 410 bore brass. NRA Member
 July 31, 2012, 10:45 AM #2 steveno Senior Member   Join Date: June 18, 2004 Location: Minden , Nebraska Posts: 1,403 buy a chronograph
 July 31, 2012, 10:55 AM #3 Brian Pfleuger Moderator Emeritus   Join Date: June 25, 2008 Location: Western Colorado, finally. Posts: 19,107 Velocities are semi-sort of-kindof-linear in a normal load work up. Generically speaking, your mileage may vary, there are exceptions, it's not perfect even when it's not an exception. Besides which, your gun WON'T match the published velocity. Almost guaranteed. Almost, again, there are exceptions. So, yeah, buy a chronograph. __________________ Still happily answering to the call-sign Peetza. --- The problem, as you so eloquently put it, is choice. -The Architect ----- He is no fool who gives what he can not keep to gain what he can not lose. -Jim Eliott, paraphrasing Philip Henry. Last edited by Brian Pfleuger; July 31, 2012 at 09:03 PM. Reason: Typo
 July 31, 2012, 10:57 AM #4 wogpotter Senior Member   Join Date: September 27, 2004 Posts: 4,811 For a reasonable approximation, draw a graph in "MS Excel". low load & low velocity at one corner high load & high velocity at the other. Join the points with a line & mark off the increments you want. __________________ Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”? Elderly Hunter: “That's dashed unsporting. Probably Belgium.”
 July 31, 2012, 12:08 PM #5 wncchester Senior Member   Join Date: December 1, 2002 Posts: 2,832 The velocities in factory ballistics charts and our loading manuals are often so far off reality that any efforts to 'calculate' a speed has nothing to start from.
 July 31, 2012, 12:37 PM #6 moxie Senior Member   Join Date: October 17, 2006 Location: TX Posts: 513 As Brian said, there is no linear relationship between powder charge and velocity, but as long as you are staying between published min and max loads you can get a close approximation, AKA, a wag. But don't do this outside the min/max realm. Pressure spikes and so forth can really ruin your day. __________________ If you want to shoot...shoot...don't talk! Tuco USAF Munitions 1969-1992 RVN 1972-1973
 July 31, 2012, 08:55 PM #7 Lost Sheep Senior Member   Join Date: January 24, 2009 Location: Anchorage Alaska Posts: 3,340 Linear is the best you are going to get without your own chronograph While the relationship between minimum and maximum charge weights to velocity at those weights is not linear there are two things that make it necessary to assume that it is. 1) The load manuals are in a pressure range where the pressure curves are fairly well-behaved. While the curve may not be linear, the velocity goes up as the charge weight goes up BETWEEN the minimum and maximum. 2) Linear is the best guess for the first approximation. Besides, you are "shooting in the dark" anyway. 3) Without a chronograph to tell you what you are actually getting (even at the published end points) your linear guess is no more unreliable than using the manuals' publishers' velocities and a linear assumption. Lost Sheep.
 July 31, 2012, 10:29 PM #8 ThisIsMySig Member   Join Date: June 6, 2012 Location: Apache Junction, Arizona Posts: 65 Thanks very much for all your responses. They all make sense. Using the min and max velocities/loads, I can see that an Excel type graph would tell me what I want to know. But like several have pointed out, a chronograph is really the only way to go. __________________ Sig P250 in 9mm and .45 cal, Sig 1911 .22LR, Heritage Arms 32 H&R Magnum revolver, S&W Governor. Reloading 9mm, 32 magnum, 45 ACP, 45 LC and 410 bore brass. NRA Member
 July 31, 2012, 11:50 PM #9 black mamba Senior Member   Join Date: September 13, 2011 Location: O'Fallon, MO Posts: 709 Also pay close attention to the barrel lengths used for the published velocities, especially in handgun cartridges. Usually 7-1/2" to 8-1/4" pressure barrels are used, so there is no barrel/cylinder gap or semi-auto action to bleed off pressure and velocity. It's not unusual for 4" to 5-1/2" revolvers to give more than 100 fps less than published velocities when actually chronographed.
 August 1, 2012, 05:31 AM #10 30Cal Senior Member   Join Date: December 3, 2002 Posts: 1,214 Linear will work fine, especially if you just need a number for a ballistic calculator.
 August 1, 2012, 05:58 AM #11 B.L.E. Senior Member   Join Date: December 20, 2008 Location: Somewhere on the Southern shore of Lake Travis, TX Posts: 2,450 I think you'll get a better match if you do a linear calculation not on velocity but on bullet energy. Take the kinetic energy of the minimum and maximum loads and draw a straigh line between the two to find the kinetic energy of the between load and then calculate velocity based on that kinetic energy. I have found that this works very well when someone posts results of several powder charges.
 August 1, 2012, 07:25 AM #12 wogpotter Senior Member   Join Date: September 27, 2004 Posts: 4,811 There's no reason why you can't combine the Excel graph & the chronograph. Shoot a maximum minimum*, a median* & a minumium maximum* & draw a line through the three points with Excel. This is how I create a load ladder with new components. This is one I made years ago matching a possible load to a trajectory compensating scope' choice for cams. I ran different powders & bullets to see what would be the closest match to the calculated cam in the range turret & tested the best calculated answer. This was drawn manually but the "scatter graph" in Excel is excellent for this type of work where you have several sets of data points in different series & you want to get a real world projection of the likley outcome. *Explaination of these terms. Maximum minimum = The highest "starting load" from several reloading manuals. Minimum maximum = The lowest "maximum load" (back off 10% if you feel the need for safety reasons) Median = 1/2 way between the high & the low. __________________ Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”? Elderly Hunter: “That's dashed unsporting. Probably Belgium.” Last edited by wogpotter; August 1, 2012 at 07:49 AM.
 August 2, 2012, 11:13 AM #13 Unclenick Staff   Join Date: March 4, 2005 Location: Ohio Posts: 13,648 You'll find the fps/grain between two points is actually quite linear provided the maximum load is not compressed and the charge doesn't cross a pressure where case expansion differences affect linearity (usually for loads that peak around the 30,000 fps range). It's actually more linear than KE is, though both are pretty linear. Once you start compressing charges, though, the air space in the case between grains is changed, and that can cause non-linear changes in pressure. For load data and commercial ammo stated velocities, these are, as mentioned, taken from standard pressure and velocity barrels. Under SAAMI standards the lengths of these are 24" for the majority of rifles (with a few exceptions, like .30 Carbine at 18"), and for handgun rounds they vary a good bit more, sometimes having two standard lengths for a single round. The SAAMI standards are available on line, and pressure and velocity test barrel specifications are in the second halves of each of these documents: Rifle Handgun Rimfire Shotgun __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member
 August 2, 2012, 11:29 AM #14 Mobuck Senior Member   Join Date: February 2, 2010 Posts: 5,163 You could also do it the old fashioned way with a pencil(and calculator if you're numerically challenged). You'll still only get an estimate.
 August 4, 2012, 04:30 PM #15 moxie Senior Member   Join Date: October 17, 2006 Location: TX Posts: 513 Thre are too many of you guys who stayed awake in math class. Cheaters! __________________ If you want to shoot...shoot...don't talk! Tuco USAF Munitions 1969-1992 RVN 1972-1973
 August 4, 2012, 06:25 PM #16 oldpapps Senior Member   Join Date: September 22, 2011 Location: Middle America Posts: 518 I see the listed velocities as only a ruff guide line and 'starting' loads as just that, a place to start. I have found that too many outside influences come into play with both the internal and external ballistics for me to be able to do more than give an educated guess. And I willingly will tell you that my guesses aren't very good. Below are the numbers I got working up a loading. The round is the .223/5.56, mixed military brass, CCI SR primers, the bullets 62 grain SS109 (not pulls) and Varget powder. All brass was full length sized, trimmed to the same length and loaded to the same overall length. The only difference was the amount of powder and I was very picky about getting that right. All fired from the same 16 inch 1 in 8 twist SS barrel. Velocities are an average of 5 shots each, with my Master Chrony Beta Model at 10 feet. All shot one after the other on the same day. All velocities are rounded to the nearest whole number. I did not have any wild velocity pop up on any of these strings. 24.5 – 2595 24.7 – 2654 25.0 – 2653 25.2 – 2689 25.4 – 2697 25.5 – 2676 25.7 – 2785 25.8 – 2750 25.9 – 2758 26.1 – 2815 26.2 – 2818 26.3 – 2842 26.4 – 2811 26.5 – 2832 Why did some velocities decrease with the addition of more powder? I don't have a clue (that counts). I did make a second run of one of the sets and yes, the velocities dropped. No, I didn't get them out of order. That was 14 baggies with the loading on a note and rounds sealed in each. Be safe, OSOK
 August 5, 2012, 03:02 AM #17 458winshooter Senior Member   Join Date: June 16, 2008 Location: Louisville,Tn Posts: 507 Ballistic Latter What caliber are you talking about,what powder,and what bullet?
August 5, 2012, 03:57 AM   #18
Scorch
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Join Date: February 13, 2006
Location: Washington state
Posts: 12,833
Quote:
 Why did some velocities decrease with the addition of more powder?
My opinion:
If you had tested 100 rounds of ammo with a given powder charge, bullet weight, primer, etc, you would have a normal distribution of velocities, with 68% of them falling within 3 standard deviations, and the remaining 32% as outliers in the tails of the normal distribution curve. What you are seeing in your 5-hot samples when you see a decrease in velocity for adding powder is outliers affecting the velocity average. You also have some relatively large increases in velocity for somewhat minor increases in powder charge, and they are just as relevant as the decrease for a minor increase in powder charge.
* Even though you trimmed and sized the brass, were they all the same lots of brass to begin with? All brass is not created equal.
*Plot the sample measurements and get a straight-line fit and you will see what your predicted velocity should have been. You are obviously intrigued by the velocity increases with small charge increases, but they may be the ones that are off the line.
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August 5, 2012, 03:29 PM   #20
Unclenick
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There's something to both sides of the argument. I put the rifle data in Excel and got:

You'll notice that velocity increase with charge weight is pretty monotonic. Using the straight line least squares fit to predict what the velocities should be based on the average trend, then finding the deviations from the prediction I got a standard deviation of 23 fps. 68% of a popluation are within 1 standard deviation, 95% within 2 standard deviations, and 99.3% are within 3 standard deviations, so only one in 300 would be expected to be more than 69 fps off the prediction made by the line. Pretty typical of a rifle.

Just one problem.

The datapoints are 5 shot averages so we would expect the standard deviations to be 5 times smaller than typical of a rifle. So 23 fps for 5 shot groups is like having 115 fps standard deviation for single shots. Not so likely, though the mixing of brass and pulled bullets probably account for some of it. Unfortunately it's tough to catch all the variables. Changing light could account for some shift in chronograph readings, as could changing temperatures (both ambient and barrel), not to mention changing fouling in the bore. But I wasn't there so I don't know what did or didn't happen with all that. It's sometimes a messy business.
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 August 5, 2012, 07:48 PM #22 ROGER4314 Senior Member   Join Date: May 17, 2009 Location: East Houston Posts: 257 As the guys stated, velocity can vary from gun to gun so nothing is exact. Velocity can also vary by temperature, barometric pressure and humidity. I take the max load, and divide it into my charge to get a percentage of max load. We'll say 94% of max charge weight. I multiply that 94% by the max load velocity. That's what I use for my personal records. That method is NOT scientific and NOT reliable but it gets me in the ballpark. I own a chronograph so if I was that concerned, I'd actually test the rounds. Flash
 August 5, 2012, 10:21 PM #23 ThisIsMySig Member   Join Date: June 6, 2012 Location: Apache Junction, Arizona Posts: 65 Not for nothing but, you guys have an incredible wealth of knowledge. __________________ Sig P250 in 9mm and .45 cal, Sig 1911 .22LR, Heritage Arms 32 H&R Magnum revolver, S&W Governor. Reloading 9mm, 32 magnum, 45 ACP, 45 LC and 410 bore brass. NRA Member
 August 6, 2012, 01:55 AM #24 sc928porsche Senior Member   Join Date: May 29, 2008 Location: now living in alabama Posts: 2,433 I have yet to have a factory round fired from my rifle to meet the advertised fps. Same can be said of the posted fps in reload manuals. Each rifle is an individual and the only way to really know is to get a chronograph. Why guess when you can know. __________________ No such thing as a stupid question. What is stupid is not asking it.
 August 6, 2012, 07:44 PM #25 tobnpr Senior Member   Join Date: August 1, 2010 Location: Tampa Bay Posts: 4,332 Finally bought a Chrono a few weeks ago. For a couple of calibers, my estimated MV based on linear interpolation of the charge/published velocity was pretty damn close. Others, waaay off. But, I kinda knew that before buying the chrono. After zeroing at one or two hundred yards, the come-ups required at 600 either verified- or shot down- the estimated velocities. As mentioned, it's the "real world" results downrange that matter.

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