Ballistic Voodoo - Uncle Sam Sez...

[InTheBlack]

These are from the new revision (2003) of FM 3-22.9, Rifle Marksmanship:

Apparently we are using rocket-assisted projectiles in the M16:
>>>
(2) Muzzle Velocity. Muzzle velocity is the speed of a bullet as it leaves the barrel, measured in feet per second. Muzzle velocity diminishes as the bullet gets farther away. The bullet reaches its maximum velocity 76 feet from the end of the rifle and slows down from there until it reaches the target.
>>>


And this just makes no sense whatsoever; maybe an example of firing up at the roof of a skyscraper got dropped from the final text:
>>>
(b) The complicating factor in shooting uphill or downhill is that the wind will affect the shot over the entire slant range. The shooter should aim at the target as if it were 25 yards away and correct for wind as if it were 400 yards away. The correct method for shooting uphill or downhill is to adjust elevation based on the horizontal range, and correct for wind deflection based on the slanted range.
>>>9

 

[InTheBlack]

And as long as I'm on the subject-- instead of explaining that an MOA is for all practical purposes one inch per 100 yards of range, which is a fact that can be used to actually help you hit your target, here's what they say:

>>>
A minute of angle is an angle beginning at the muzzle that would cover 2.54
centimeters at a distance of 91.4 meters (Figure 5-37).
>>>

Now I'm going to take a break and drink 355ml of beer.a

 

[PenHolder]

Hmm, let's see. *brushes cobwebs from brain*

A minute of angle is an angle beginning at the muzzle that would cover 2.54 centimeters at a distance of 91.4 meters (Figure 5-37).

Unless I'm mistaken, 1 M.O.A. is 1/60th of a degree. tan(1/60 deg) is ~0.0002909; that's the ratio of the deflection amount to the distance to target.

So, tan(1/60 deg) * 91.4 m * 100 cm/m = ~2.6587 cm of deflection at 91.4 m, not 2.54 cm.

What I don't understand is, if they're going to be so precise as to carry these things out to three signficant digits, why they don't at least make sure they use the correct digits?

Then again, why not just say, "2.91 cm at 100m", with one less odd-looking figure to remember? Or, for the metric-impaired, "1.05 in at 100 yd"? If that extra 20th of an inch won't make much difference -- and at say 500 yd, that's a hair under a quarter of an inch -- who cares, when you're shooting at enemy troops?

-PH

The metric system is the tool of the Devil! My car gets fourty rods to the hogshead, and that's the way I likes it! -- Abe Simpson

 

[swampsniper]

Where does the energy to accellarate the projectile after it leaves the barrel come from? If we could figure this out, think of the limitless potential here. Could they be approaching perpetual motion here? Are there actually any military instructors out there that will teach this voodoo?:banghead:

 

[Al Thompson]

Looks like another FM written by a committiee. Between the duds at Ft. Benning and TRADOC, much usefull info gets ignored. My last tour in the Army I reviewed the manual for the M2 .50 cal. That was an exercise in futility. It's sort of like anyone with a clue is excluded from producing the manual.

 

[Art Eatman]

swampsniper, you took the words right out of my mouth.

That's unsanitary.

You learn lots of weird things from the military folks. In Basic Training, the super expert Sergeant who was explaining the Garand to us Newbies seriously described it as a "...gas-operated...shoulder weapon." He went on to tell us that the chamber pressure was 50,000 psi. This meant 25,000 psi to propel the bullet and 25,000 psi to push the bolt back to reload...

That was one of the few times in my younger years that I was smart enough to stay shut up.

, Art

 

[Richardson]

units

Penholder,

What they did is translate the translation, so to speak.

The authors of this training material were probably instructed to "use metric." Instead of calculating "from scratch", they just took the english unit approximation of 1" at 100 yards and translated each into metric.

1 inch ~> 2.54 cm
100 yards ~> 91.4 m

Unless the revisers of this document really understood the math (and I doubt that they did), they had no way to know how to calculate this for 100 m.

The same problem comes from tranlating a translation. The example I'm most familiar with is with the Bible. The Roman Catholic Church named the Latin Vulgate (a translate of the "original" Hebrew & Greek) the "officially correct" Bible. Translations into other languages then began using the Vulgate instead of the Hebrew & Greek Manuscripts. Obviously, this led to a less accurate translation. Some people today use the KJV bible as their basis for translating the bible into non-english languages.

If you were ordered to "convert the manual to metric" with no technical understanding of MOA, and no one to ask the technical question, what would you do, "private"?

Richardson

 

[MrAcheson]

My guess is that the extra speed comes from gravity. The bullet is constantly being accelerated by gravity and decelerated by air resistance. Its possible that in the short range gravity may be speeding the bullet up more than air resistance is slowing it down. Sounds fishy to me though.

 

[swampsniper]

You wanna have some fun go around and ask them what "point blank" means. It is so strange that so many victims choose to stand right there!! The last member of this family to go through basic was my Daughter. When she was able to make Her first call home, she said," Daddy, the range is a joke". This is a kid that made head shots on squirrels before she hit first grade.

 

[swampsniper]

mracheson, I just reread your post. Gravity would only add velocity if the barrel were pointed below the horizontal, and not much at that. Velocity is properly measured from a horizontal bore, just far enough from the muzzle to prevent problems from muzzle blast, usually 15 feet. In reality, velocity ceases to increase when all powder is consumed and pressure reaches max. If you had a light charge, projectile can actually reach max velocity before exit, and slow down before exit.
Test this please, if you take your foot off the gas while accelerating, does your forward speed continue to increase? Level road, of course.
Velocity loss is actually greater closer to the muzzle than somewhat down range, the effects of air resistance are not a simple function.
Whole point is, if we are trying to train riflemen we should make them as familiar with their rifle as they are with their own body. Misconceptions have no place when you have to fight for your life. Individuals that allow bad info to be used have no place teaching anything.

 

[HankB]

1. Even fired straight down, the bullet will slow upon leaving the muzzle - air resistance at rifle velocity greatly exceeds gravitational effects.

2. Velocity continues to increase in the barrel as long as the force on the bullet base exceeds the frictional resistance of the barrel and the resistance of the mass of air compressed ahead of the bullet. This acceleration will normally continue well past peak pressure, which is reached during the bullet's first few inches of travel. Pressure at the muzzle will still be somewhere around 10,000 PSI, give or take a bit, depending on cartridge, powder, etc.

3. I've no idea where the "76 feet" came from in the original post. Before electronic chronographs, the military used mechanical systems to measure velocity at 78 feet . . . but even then, a simplifying assumption was made that velocity falls off linearly with distance.

 

[InTheBlack]

Thanks for confirming what I thought I recalled about where velocity was measured. I was thinking somewhere around 25 yards. Obviously, whoever wrote the manual doesn't have a clue about what he's writing about. The statement that muzzle velocity is measured 70-something feet from the muzzle got transmogrified into the crap in the manual.

Honestly, I don't understand how this manual hit the street.

The tables for bullet drop at various ranges are impossible to actually use to quickly figure out how to hit a target; sometimes you have to presume that they are based on a 300 yard zero. If this is the "text" from which people are trained, the trainers have a lot to do.

Nowhere did I see an explanation of why the elevation wheel has various numbers on it. That's pretty basic.

No discussion of point blank zero or the beaten zone either. It was important enough to explain in the 1942 edition, why not now?t

 

[griz]

What Richardson said about literally translating measurements is true. I saw one report that talked about “a distance of approximately 304.8 millimetersâ€. What they meant was one foot, give or take half and inch. The translation comes out looking stupid.

They are right about wind drift while shooting at large elevations. Even if they explained it in such a way as to make it sound wrong. The only thing I can figure about the velocity thing is somebody was drinking on the job.