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Muzzle Velocity... WHAT??

3K views 11 replies 10 participants last post by  Big Shrek 
#1 ·
I know that ammo manufacturers post muzzle velocity on their packaging, but how is that number calculated?

For Example:

I have a Taurus 17HMR revolver with an 8.5 inch barrel. There are a few different choices on ammo and they all list the muzzle velocity at around 2550 fps. I have also been told that barrel length is a major factor in this equation. Am I to believe that my 8.5 in barrel will produce the same muzzle velocity as say a 21 inch rifle in the same bore?

Not trying to start trouble with this question, just trying to learn more about something that interests me.

All information is appreciated!
Thanks!
 
#3 ·
it's possible , that a bullet will reach it's peak muzzle velocity at 8.5 inches and nothing will be gained with a longer barrel. not sure that is the case with the 17HMR but there are some calibers in which this is the case. one example comes to mind. when hornady was testing their new critical defense ammo a 3 inch barrel and a test barrel of 8 inches was used. from the 3 inch barrel the velocity was almost the same as the 8 inch barrel. does seem hard to believe but it's factual. seems to me i recall reading a test on some 9mm ammo being tested in a H&K MP5 vs. a sig 226 and the numbers matched for muzzle velocity
 
#4 ·
Manufacturers post a muzzle velocity from a test barrel. Those test barrels can be from 18 to 24 inches for rifle ammo and usually 4 to 5 inches for pistol ammo, depending on the manufacturer.

They have no way of knowing what your barrel length is. Of course, as a general rule, a longer barrel will allow for a greater velocity (to a certain point).

17HMR would be considered a rifle round and was probably fired from a long test barrel. You're not likely going to get close to 2550 fps out of a pistol with it. Not sure what its actual ballistics are in that barrel length.
 
#5 ·
They calculate the MV with a device called a chronograph. Think of it as a high speed version of a cops radar gun. you shoot over the device and the sensors (2 of them) record the bullet passing over and calculate the time difference into FPS. Most factory ammo is tested with a rifle length test barrel so velocities will be considerably lower with short pistol barrels. One example, My Ruger single six .22 mag shooting Hornady 30 gr Vmax at an advertized velocity of 2200 fps will actually produce right at 1600 fps (I do have a chrony) my ruger wears a 5.5" tube, and the .22 mag ammo was likely tested in a 24" tube. Make sense?
 
#6 · (Edited)
I know that ammo manufacturers post muzzle velocity on their packaging, but how is that number calculated?

Thanks!
Well, it isn't calculated, it's measured over a chrony as the other posters have stated. All MVs are empirical data. I'm pretty sure there are differential equations one might gin up, and maybe the mfrs have done so for when they start designing a new loading. Variables include rate of powder burn, friction between the projectile and test barrel bore, weight of projectile, pressure developed in the gas during the time it takes for the bullet to clear the bore, rate of pressure buildup (and in your case, cylinder to throat gap) and others I can't imagine. There are some general rules that are not particularly accurate, but close enough for field use. Use 100fps reduction for each inch of barrel shorter than the test barrel. You can't make the same assumption for longer barrels, because of gas expansion slowing down as the containing cylinder volume (barrel enclosed by the chamber and moving bullet) increases, but adding 100 for each additional inch, up to say 3" won't be too far wrong.

Ballistics internal to the barrel is extremely complex, and when you come down to it, irrelevant to what you need to know in the field. External ballistics are known to a gnat's sphincter because they can be directly measured. Please forgive me for overstating the obvious--
 
#7 ·
The loss/gain per inch of barrel is usually not that high, running 25-50 fps as a rule of thumb. As to the length of barrel used in the factory testing, the only way to find out for sure is to ask them.

There is something of a myth about when a bullet "runs out of steam" and can actually slow down in a barrel due to friction. The origin seems to be the true statement that the powder in a .22 LR case is fully burned by the time the bullet has moved 16". Some folks conclude then that the bullet slows down from that point and is coasting on momentum with no pressure behind it. But this idea ignores that fact that the hot gas generated by the burning powder continues to expand and generate pressure after the powder itself has all been consumed. (If there were no pressure past 16", .22's with longer barrels would not make any noise, since the noise comes from the release of pressure on bullet exit.)

Jim
 
#8 ·
JimK:

I don't think it is a pure pressure thing or the time at which all the powder is consumed. At some barrel length the pressure subsides** to the point that it can no longer over power the friction of jamming a tight fitting bullet down a tube. The result is the bullet slows. Myth or not (???) it is pretty common knowledge for instance that a 22LR bullet looses velocity if used in a gun with a barrel much longer than about 16 inches. Since I have not tested it myself I can not verify it.

** The expanding gases have an ever increasing volume to fill as the bullet goes down the barrel. Eventually the pressure pushing the bullet, which now fills a bigger volume, diminishes to a critical level where it can not totally overcome the friction of the bullet and acceleration of the bullet stops.

I think that's the way it works (??).

LDBennett
 
#9 ·
JimK:

I don't think it is a pure pressure thing or the time at which all the powder is consumed. At some barrel length the pressure subsides** to the point that it can no longer over power the friction of jamming a tight fitting bullet down a tube. The result is the bullet slows. Myth or not (???) it is pretty common knowledge for instance that a 22LR bullet looses velocity if used in a gun with a barrel much longer than about 16 inches. Since I have not tested it myself I can not verify it.

** The expanding gases have an ever increasing volume to fill as the bullet goes down the barrel. Eventually the pressure pushing the bullet, which now fills a bigger volume, diminishes to a critical level where it can not totally overcome the friction of the bullet and acceleration of the bullet stops.

I think that's the way it works (??).

LDBennett
That's the way I've always understood it, LD. I hope we're not getting too far off base and confusing the OP.
 
#10 ·
There are two good sources that I know of talking about how long a barrel is compared to the caliber & the capacity of the case. One is the NRA amunitition & firearms fact book & the other is Hatcher's notebook.

To take the .22 LR as an example, a barrel 20 inches long is 90 calibers long. This is a pretty long barrel. Depending on the brand & type of .22 LR ammo max velocity is gotten in 14-19 inches of barrel. After that point the pressure is lower than the forces exerting to slow the bullet down including friction & moving the air in front of the bullet. After 14-19 inches of travel the bullet will be decelerating even though there is some pressure left in the barrel.

A 22 LR barrel was tested at 18.5 inches & 28 inches with a particular brand of ammo & velocities were 18.5 inches=1467fps & 28 inches=1338fps.(from the NRA Fact Book).

Cases with more capacity & different types of powder can keep bullets accelerating thru longer barrels. Longer barrels are sometimes chosen anyway for reasons of asthetics or lengthening the sight radius in calibers like the .22 LR that aren't optimal for velocity.

Some barrels are shorter than needed for optimal velocity for reasons of handiness.
 
#12 ·
One other thing not mentioned...

Different types of barrels get different muzzel velocities.

Take a ballard barreled Marlin 336 (current) and a Micro-groove Marlin 336 barreled from a few years back.
If you take the time to fit a particular bullet to the micro-groove barrel, you end up with a better gas seal,
which causes slightly faster velocities thru a Chrony. Average is 100-200 FPS faster in .30-30

However, it is a bit of work...much of it done with a Micrometer, checking each type of bullet
and finding the fattest bullets you can to better work with the micro-groove barrel.
This will, however, make standard ammo work very much like Match-grade ammo, as
fitting a specific ammo to the barrel give you a much better shooting solution.

Why buy a match barrel when all it takes is careful ammo selection?? :D
 
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