The M16 design has been altered and enhanced time and time again. Utilizing the inherent accuracy and the unmatched modularity, the M16 has been turned into an excellent sniper and counter-sniper weapon.
Squad Designated Marksmen (SDM's) are becoming more and more common and they are often using slightly modified M16A4's with scopes (often an ACOG), bipods, 2-Stage triggers and other things to enhance accuracy. They don't act as snipers in the true sense of the word, but they do act as a more precise longer range rifleman.
The SPR (Special Purpose Rifle), also known as the Mk12 as named by the USN, is another M16 variant. Featuring an 18" free floating match grade stainless steel heavy barrel, the SPR is usually fed with 5.56 Mk262 Mod 1 77gr OTM (Open Tip Match) Black Hills rounds. This load is known not only for it's supreme accuracy but also for devastating fragmentation (when traveling at or above the fragmentation threshold of 2300fps).
A KAC (Knights Armament Company) 2-Stage trigger is used, as is a PRI free float tube and a Bi-pod. A typical scope is a 3.5–10x40mm Leupold.
There are some variations out there in the SPR family, including a variety of stocks, but you get the picture. Many AR15 enthusiasts make their own versions of the SPR based on the various features I mentioned.
Similar to the SPR, the SR25, or Mk11 Mod 0, made by KAC is made for use by snipers. However, this rifle is chambered in 7.62x51 NATO.
This picture shows the difference between the 5.56 (top) and the 7.62x51 (bottom).
The SR25 uses 20 round magazines along with match ammo.
A USMC Marine is shown shooting the Mk11.
The Mk11 Mod 0, USN and USMC version, is very similar to the M110 Semi-Automatic Sniper System (SASS) of the US Army ,and both are made by KAC.