The M4 rifles the military uses in Afghanistan are what caliber? Why did they choose this ammo? Snipers use?

the M4 uses 5.56 NATO the snipers use 7.62 NATO

These questions are a bit more complex than you think.

Way back during the Wild West the military used the 45-70, a big bullet capable of dropping a grizzly bear that could fly over a mile but had a long arching trajectory so it wasn't terribly accurate at even moderately long ranges. Then some guy invented a newer type of gunpowder and another guy invented a pointy bullet. The USA found it's older stuff to be no match for this modern stuff, so we copied it.

Enter the Springfield 1903 rifle and the 30 caliber 1903 ammunition...which was updated to an even better shaped bullet in 1906. This is what we get 30-06 (say it thirty - aught - six) from.

We used in World War One and World War Two.

These rifles and 30-06 ammo had sights set up for shots for as far away as 2850 yards which is more than a mile and a half away.

During World War Two we learned that it isn't very often a soldier needs to shoot at something a mile and a half away. Heck, just at half a mile away a person is just a dot.

However the military didn't act on this knowledge right away.

What they DID do after Wold War Two is switch to automatic rifles and redesigned a newer slightly smaller 30-06. They named it 7.62 NATO and used in in M14s and M-60 machineguns.

Because it was already in military use, when the military decided to get new sniper rifles, the chose 7.62 NATO for the chambering.

Back in the Vietnam war we were carrying these same M-14 rifles around. In the jungles of vietnam we were ALSO not shooting at people a mile and a half away.

Then the Air Force went and bought some M-16 rifles because they were light weight. They had this new fangled 5.56 ammo, it wasn't very long range but who cares? It wasn't very powerful...but it was powerful enough. Also if you carried a pound of extra ammo you had twice as many shots as with a pound of the old stuff.

The Army eventually copied this and got their own M-16. They used it pretty hard and found it wasn't so great, so they had some changes made to the M-16 to make it stronger (M16A2), but they liked the fact that a grunt could carry more ammo so he could fight longer. And they also were fine with the lower power because they finally wised up and realized that you don't need ammo that can kill a grizzly bear to kill another soldier...and that we weren't shooting at enemies who were a mile away.

That's why the military stuck with the 5.56 NATO ammo. Later the army went with an even smaller and lighter verson of the M16 and named it the M-4 and kept the same ammo for it.

M4 rifles are chambered .223/5.56 nato.

They use it because its low recoil and still powerful enough to get the job done.

I believe Snipers use .308 .338 lapua and .50 bmg in their rifles,they might also have a few others,Im not 100% sure because im not a sniper.

Contrary to what you see in games and movies,snipers rarely use .50 bmg to kill people,its more to stop vehicles

I may be wrong,If I am some one with more knowledge on this subject will correct it.

the M4 uses the 5.56x45mm NATO round. it is used because it is a high velocity round with severe wounding capabilities. i believe the US military uses the 7.62x51mm NATO round for its sniper rifles

5.56x45mm NATO(similar to a .223 remington), due to it's light weight, you can carry more of it compared to large rifle cartridges. snipers use the 7.62x51mm(similar to a .308 winchester), .338 Lapua Magnum, .408 Cheytac. their anti materiel rifles(often mistaken for sniper rifles) are chambered in .50 BMG.

The .22 Colibri's because they are quiet