The U.S. Marine Corps has followed the U.S. Army and ordered CSASS (Compact Semi-Automatic Sniper System) rifles to replace its older M110 sniper rifles. Compared to the M110 the CSASS is shorter and lighter than the M110 and army snipers told their marine counterparts that this made a big difference in the field because in terms of accuracy and reliability CSASS was at least as good as the M110 but the shorter length and lighter weight made it a lot easier to get into and out of shooting positions. The Marines have ordered 116 CSASS rifles at $8,595 each. The marines are also going to replace their more numerous M40 sniper rifles with the Mk 13 Mod 7 sniper rifle. The main reason for this is the Mk 13 uses the more powerful .300 Winchester Magnum round and makes longer range (over 1,000 meter) shots possible. The Mk 13 is widely used in SOCOM, especially by SEALs.
Meanwhile, the rest of the army has been looking for something that is easier to use. Back in 2016 the army introduced a new model of the CSASS sharpshooter rifle to supplement the first one that entered service in 2012. The army ordered 3,643 of the 2016 CSASS which is based on the German H&K G28. This is a 7.2 kg (15.8 pound, with loaded magazine), 965mm (38 inch) long semiautomatic 7.62mm rifle with a ten round magazine. It is gas operated with a Picatinny rail for mounting the scope and other accessories. Compared to the 2012 CSASS the G28 is easier to handle and has reduced recoil, a better suppressor and improved pistol grip and stock. It is believed to be more reliable and easier to maintain.
The 2012 CSASS was a shorter and lighter version of the M110 SASS (Semi-Automatic Sniper System) that was introduced in Iraq and Afghanistan in 2008. The CSASS used a collapsible stock and a removable flash suppressor to get the length down to 91 cm (36 inches) and the weight under 4.1 kg (9 pounds unloaded). This was done by modifying existing M110s. The CSASS is addressing demands from the troops for a more compact sniper rifle for the many snipers who are part of infantry units, not special sniper detachments.
The M110 has largely replaced the bolt-action M24 as a sharpshooter weapon and provided commanders with much more effective snipers. That increase in numbers (of snipers) and their effectiveness has changed the look (less random fire from U.S. troops) and feel (the U.S. troops appear more in control) of the battlefield. It's also easier to spot the enemy using lots of snipers (or sharpshooters). The enemy is usually the guy firing on automatic. The fellows firing one shot at a time are the Americans and they are usually the last ones standing.
The M110 was not itself a big technological breakthrough. It is based on the older AR-10 rifle. The U.S. Navy had already been buying a similar weapon, the SR25 (also known as the Mk11) since the early 1990s. All of these are 7.62mm semi-automatic sniper rifles based on basic designs developed by former marine Gene Stoner in the 1950s. This led to the M-16 and many other rifles. The basis for the M-16 was the AR-15, and a 7.62mm version of that weapon was called the AR-10. About half the parts in the SR25 are interchangeable with those in the M-16.
The M110 achieved its high accuracy partly by using a 50 cm (20 inch) heavy floating barrel. The "floating" means that the receiver (and barrel) is attached only to the main body of the rifle. This reduces resonance (which throws off accuracy) after each round is fired. The M110 weighs 7.9 kg (17.3 pounds) in combat and about 32 kg (70 pounds) with all components of the system. The M110 can use a ten or twenty round magazine. The 1.03 meter (40.5 inch) long rifle can have a 15 cm (six inch) tube attached to the barrel, which reduces the noise and flash made when the rifle fires and largely eliminates nearby dust rising into the air, which often gives away the sniper's position.
Previously, many snipers have had success using tuned up M-14s (from the 1960s) as sniper rifles. While semi-automatic and rugged, the M14 wasn't designed to be a sniper rifle. The AR-10 was a better model for a semi-automatic sniper rifle since it is inherently more reliable and accurate. And that was just the beginning as improvements in sniper rifle design have become the standard since the 1990s.
As far back as World War II it was known that there were many situations where a semi-automatic sniper rifle would come in handy. But it's taken over half a century to solve the reliability and accuracy problems. The CSASS is also meant to satisfy users of the M-14s, whose size (118cm/46.5 inches, same as SASS) was always a drawback. The SASS, however, weighed a third more than the M-14. With the lighter and shorter CSASS, plus a reputation for accuracy and reliability, M-14 users were finally lured away from their 1950s era sniper rifle.