This field manual is often quoted on internet forums, probably because it is everywhere available at army surplus and so-called “tactical” stores. But it is wrong about a lot of things. Just because a document was prepared by the U.S. Army, you should not assume that the author knew what he was talking about. If a store’s principal income is from t-shirt sales, do not trust the books they have for sale.
In fact, I strongly suspect that the field manuals that were released to the public in the 1960s were planted to mislead civilians. FM-23-10 has an air of truthiness, but some of them, especially those supposedly written for the Special Forces to train indigenous guerillas, are just straight-out bullshit. The one about improvised incendiary devices is; the joke is on you if you set yourself on fire trying to make napalm by heating soapy gasoline on your kitchen stove. Duh! Also, putting linseed oil in an automobile fuel tank has no effect. I think these were released during the Vietnam War to monkey with anti-war demonstrators; sabotage at National Guard bases was a problem back then.
Modern snipers do not cite FM-23-10. The people citing it are invariably wearing black t-shirts that read, “If you run, you will only die tired.” More bullshit! Hitting a moving target is difficult; running, especially laterally, is always a good idea. In fact, the only way to shoot more than one enemy with a bolt-action rifle is to engage them near cover that is not bullet proof, like trash cans. They hide and then you just methodically shoot through each trash can. Note. If you had engaged them in the open, they would have scattered and you would have never gotten a second shot off.
FM-23-10 (Ch. 3, p. 36) writes:
Remember, if humidity goes up, impact goes down; if humidity goes down, impact goes up. As a rule of thumb, a 20-percent change will equal about one minute, affecting the point of impact.
They have got it backwards; humid air has less drag because H2O replaces N2 in the atmosphere and the former molecule has an atomic mass of 18 and the latter molecule has an atomic mass of 28. Also, they greatly exaggerate; a 100% change does not equal even a quarter minute of angle. When developing my Android app for mortar fire control, I painstakingly calculated the effect of humidity on mortar fire and found it to be less than one meter for shots on targets several kilometers away. Nobody can aim a mortar with such accuracy that they would care, so I just omitted humidity from my calculations. The same goes for snipers: just ignore humidity.
Factors Affecting Range Estimation
Distance makes it difficult to see things and other factors, like fog, also make it difficult to see things. It may not be possible to separate these two factors in one’s mind, so large, well-defined objects that contrast with their background will appear closer than small irregularly-shaped objects that blend into their background or are obscured by tree leaves. This is the principle reason why there are so many deer hunters around who boast of 500-yard shots on deer when, if asked to demonstrate this fantastic skill at a 500-yard rifle range, cannot even get on the paper.
FM-23-10 (Ch. 4, p. 36) writes, “The more clearly a target can be seen, the closer it appears.” So they seem to understand this point. But they also write:
Looking downhill, the target appears farther away. Looking uphill, the target appears closer.
This is exactly the opposite of what machine gunners are taught. I think the machine gun training is right; it is generally easier to see thing when looking downhill than uphill. The exception would be if the enemy is silhouetted against the sky, but this seems a bit unlikely.
FM-23-10 is not all bullshit, like some of the field manuals that have been released to the public, but some of the material is wrong and what is right is out-dated. It claims to have been written in 1994 but, if this is true, it was assembled from information compiled during the Vietnam War, thirty years earlier. There is nothing of value in jungles and there is no reason to think that we will ever fight in one again. Only cities are valuable, especially deep-water port cities, and it is there that any real fighting will occur in the future. FM-23-10 has next to nothing to say about urban combat.