The hallmark of the infantryman is the rifle. A rifle is a vastly superior weapon than a pistol as a distance weapon and for its firepower and lethality.
Rifling (lands and grooves) in a barrel is what makes it twist the fired bullet as it travels down the barrel and continue to twist in flight, stabilizing and greatly improving accuracy.
1:16 means the bullet does one complete rotation inside the barrel as it moves 16 inches down the barrel. This is called the twist rate:
Ideal twist rate is determined by the caliber, bullet length, weight and other factors. There are a lot of factors and variables at work for a sniper to be able to accurately place an effective bullet.
US military, like all modern military forces teaches 'Basic Rifle Marksmanship' (BRM); this is the foundation of military training and primary weapon used on a battlefield. Notice I'm using the word 'weapon' as opposed to 'firearm' generically, because the military and police application of these firearms is as a 'weapon'.
The basic, standard military caliber in the United States and all of NATO (North American Treaty Organization) is the 5.56x45mm NATO cartridge.
For some personnel that serve in combat units (and as snipers in special police units) the most popular caliber is 7.62x51mm NATO. Typically, military members that carry these larger 7.62x51mm rifles are 'designated marksmen', they are able to engage enemy targets at a much greater range (~800m), whereas the 5.56x45mm effective range is about half that distance (~400m).
Most designated marksmen are using 5.56 or .308 (7.62x51mm), whereas most dedicated sniper teams are using 7.62x51mm. When they may need longer range, more power or counter-sniper work the 300WM is the workhorse used and generally the upper limit of sniper operations.
The basic difference between a 'designated marksman' and a sniper is that designated marksmen stay with and support their unit as an "integral member of the infantry platoon, providing a niche capability", and a sniper generally commands strategic positions, shoots 500m or longer and has extensive training on camouflage and fieldcraft.
Here's where it really begins to get interesting. There are some even more specialized marksmen that sometimes use even more powerful rifles that have sufficient accuracy over even greater ranges. Some popular specialty sniper calibers include:
- 7.62x67mm (.300 Winchester Magnum a.k.a 300WM)
- 6.5 Creedmoor or 6.5 CM
- .338 Lapua Magnum or .338 LM
- .50 Browning Machine Gun (.50 BMG, 12.7×99mm NATO)
- 300WM, primarily a military sniper and counter-sniper caliber. Not so much used by the police as it's over powered for most police applications. It is a very popular hunting caliber.
- The 6.5 CM is the latest 'best' all around sniper caliber, winning many competitions. Although its performance offers better ballistics and it is very popular caliber, it is relatively new and some feel it may fall out of favor.
- The .338 LM, is a very powerful, flatter trajectory, successful and popular sniper and counter sniper caliber. The .338 LM shoots a very expensive cartridge and is essentially too powerful for many anti-personnel purposes.
- The .50 BMG has been adapted from a machine-gun caliber, to a very powerful long range anti-material cartridge. On rare occasions it is used for some specific sniper/counter-sniper missions overseas.
That's the overview, the .308 (7.62x51mm) is still the most popular sniper cartridge.
If you are interested in some discussion on how dedicated marksmen or snipers are used, or how counter-snipers are different, let me know for a possible future topic. This is not a particular area of expertise for me, but I know several folks that are and its a fascinating subject.