Chrono at 10' vs MV

[alloy]

When establishing a velocity at 10 feet via a chronograph, how much difference is there between that velocity and MV?

Meaning....how do I tune up a load to a spec published in MV, at the
10' distance?

For instance: how to reach 850MV 230 grain ball, .45 auto, and I'm at 810FPS at 10'

[InigoMontoya]

Pretty easy to determine *IF* you have a drag coefficient (because it WILL differ from bullet to bullet)...

Back of the envelope (I'll work in metric because it really is an easier system):

MuzzEnergy = KineticEnergyMeasured + WorkLost

WorkLost = Force * Distance

Distance = 10' = 3.0 meters

Force = Drag = 0.5 * rho * Vel^2 * Cd * RefArea

rho = DensityOfAir = 1.225 kg/m^3 (adjust according to local conditions)
Vel = 810 fps = 247 m/s
Cd = ?? ...Bah, we'll call it 0.5 (feel free to get "real" numbers)
RefArea = pi/4*Diam^2 = 3.14/4*0.45^2 = 0.159 in^2 = 102.6e-6 m^2

Drag = 1.92 Newtons.

So... WorkLost = 1.92 * 3 = 5.75 Joules

Going back to the beginning....

KineticEnergyMeasured = 0.5 * mass * Vel^2

mass = 230 gr = 0.0723 kg

KineticEnergyMeasured = 2.205 kJ

MuzzEnergy = 2205 J + 5.75 J = 2211 J

MuzzEnergy = 0.5 * mass * MuzzVel^2

=> sqrt(4422 / 0.0723) = MuzzVel

MuzzVel = 247.3 m/s = 811 fps


Now, that's a pretty sloppy methodology that could easily be improved with the use of a spreadsheet and integration methods, but it's close enough to give a pretty good idea of what's going on in those 10 feet.... And it looks like the answer is "not much."

[alloy]

Fantastic much appreciated! Really....sorry to make you type all that.

I'll go with one tenth of a grain and call it a day.

[Brian Pfleuger]

Actually, the problem that you're likely to have with the chrony 10 feet from the muzzle is a velocity in excess of muzzle velocity.

When the bullet first exits the barrel, it effectively "uncorks" the very high pressure gas behind it. The gas then expands VERY rapidly and can actually push the bullet to beyond muzzle velocity.

I have been told that a chrony distance of approximately 25 feet will actually give true muzzle velocity because the bullet will have lost that extra little "boost" at that range.

[Lost Sheep]

Wow!, Inigomontoya, that's a lot of arithmetic! And no small amount of math, either.

Would this be easier?

After recording velocities at 10', set up the chronograph at 20' and fire one or two more strings of a load previously clocked at 10'. Then determine the velocity loss factor from 10' to 20' and apply that factor (in reverse) to calculate velocity at 0'?

1100 fps @ 10'
1075 fps @ 20'
velocity loss factor: (1075 / 1100) = 0.977272727
1100 / 0.977272727 = 1125.6 fps

But, to answer the original question: I suspect that published data is not actually measured at the muzzle. Too much turbulence, smoke, unburnt powder at the muzzle. I have never seen anyone at the range set up their chronograph closer than 8-10 feet, and for more powerful cartridges, further out than that.

If anyone in the gun business knows for sure the answer to this question, "Is muzzle velocity really measured at the muzzle, or is it measured elsewhere and calculated back, or is it measured a standard distance from the muzzle and considered close enough?" Maybe SAAMI knows?

Lost Sheep

[alloy]

Now we're in a ballpark that will let me play.

[Mal H]

Originally Posted by Lost Sheep

I suspect that published data is not actually measured at the muzzle.

I suspect you are absolutely correct. If the OP is trying to determine the difference between his observed muzzle velocities and published muzzle velocities, and if he interpolates velocity back to the actual muzzle, then I'm fairly certain he has made the comparison more invalid than comparing MV's at a 10' distance.

[alloy]

Well then...this one's toast. Thanks all.

[InigoMontoya]

LostSheep, I'm not in the gun industry per se, but I am involved with tests that involve guns (guns are tools to us, not the end all). We're VERY cognizant of where the measurement was made. If we speak muzzle velocity, we backtrack the velocity as I did in my first post in this thread (albeit using more rigorous methods). We also make measurements at the target. And sometimes we track the projectile with high speed photography and make half a dozen different determinations through the projectile's flight path.

[Lost Sheep]

Originally Posted by InigoMontoya

LostSheep, I'm not in the gun industry per se, but I am involved with tests that involve guns (guns are tools to us, not the end all). We're VERY cognizant of where the measurement was made. If we speak muzzle velocity, we backtrack the velocity as I did in my first post in this thread (albeit using more rigorous methods). We also make measurements at the target. And sometimes we track the projectile with high speed photography and make half a dozen different determinations through the projectile's flight path.

I suspected that such intensive investigations were done, but had no idea and (by your disclaimer) still have no certainty that the velocities published in the loading manuals (and on the boxes of factory-loaded ammo) are given that treatment, or just taken a short distance from the muzzle and reported as read.

For my own purposes, I don't care what the velocity at the muzzle is. Because I am comparing my loads to others of my loads, usually with the same bullets (same ballistic coefficient). And I use the same distance from the muzzle all the time. So, if all my velocities are off by the same percentage consistently, I could not care less. (The story would be considerably different if I were shooting long distances and velocity, trajectory and velocity loss were significant, but I am not there yet.)

The original poster seemed to be trying to precisely duplicate a factory load (and if memory serves, 850fps 230 grain ball is the original military specification in 1906? for the 45 ACP).

That said, alloy, if you do succeed in reproducing the exact velocity you seek, it still may not exactly reproduce the factory load. External ballistics would be the same, but internal ballistics may be a little different. A little more of a fast powder or a little less of a slow powder may give the same muzzle velocity, but inside the gun, the pressure curve over time will be different, resulting in more or less residence times in the barrel for one powder over the other. That, in turn will affect how much muzzle rise the gun will experience before the bullet leaves the barrel, which will affect the point of impact at the target.

Small differences, to be sure, but I have no idea how precise your needs are.

alloy Well then...this one's toast. Thanks all.

Not so fast, there, alloy. Interesting threads like this one seldom die quickly. While some fade fast, some live on for years, resurrected from time to time, like whales sounding for a while, and then, "Thar she blows!". They reappear.

Good Luck.

Lost Sheep

[alloy]

The original poster seemed to be trying to precisely duplicate a factory load (and if memory serves, 850fps 230 grain ball is the original military specification in 1906? for the 45 ACP).

Yes that's it, in fact I'm up early to get to the range before a crowd set in.

I'm well under a max load which is 8.0 grains of sr-4756.
I've currently been shooting 7.4 grains and now that the range dried out enough for a tripod I thought I would eek the load up a bit to reach the desired original velocity, since I don't think I will be reaching that max to do it.

When I looked up the spec, I saw it was(at the first source anyway) published as a muzzle velocity.

Quote:
Originally Posted by Lost Sheep
I suspect that published data is not actually measured at the muzzle.

I suspect you are absolutely correct.

From those two things, I gather if I get up to 840-860 at 10 feet, I am generically close enough.

Not so fast, there, alloy.

Any and all discussion appreciated of course.
I figure the chrony is a tool as well as a measurement device, a couple of my loads last fall weren't near what I wanted(most were)....one 38 special plinker in particular. Right out of the book...650 fps

A few loads were surprising, and published data is sometimes amazingly accurate, and sometimes amazingly inaccurate.

[Lost Sheep]

Originally Posted by alloy

A few loads were surprising, and published data is sometimes amazingly accurate, and sometimes amazingly inaccurate.

Remember also that, while the 45 ACP is pretty much standardized with a 5" barrel out of a 1911-style platform, most other cartridges are tested for loading by the various publishers out of a WIDE variety of firearms, from box-stock over-the-counter guns to sophisticated pressure barrels with many kinds of instruments to measure who-knows-what. Some pressure barrels have vents specifically to emulate the barrel-cylinder gap in revolvers.

You might be interested in the tests conducted by a trio of guys (I think there are three of them) whose results are posted on a web site, "Ballistics by the inch"

Happy Easter

Lost Sheep

Good night

[Jim Downey]

Well, there were three of us initially, then we had a fourth join us for the second round of testing last spring.

And just as a point of reference (explained in our site protocols), we used a pair of chronys, the first at exactly 15' from the gun rest, the second immediately behind that.

Cheers!


Jim D.

[jtmckinney]

Thanks For This Post because it has led me to "Ballistice By The Inch". It is late now but I can tell that it is good stuff and I am going to read and study over the next couple of days. I have been thinking about creating a post about powders, bulk density of powders, cartridge powder capacity and barrell lengths and I think this will make for better questions.

Later,
James