The Modular Sniper Rifle, or MSR, is a bolt-action sniper rifle developed and produced by Remington Arms for the United States Army. It was introduced in 2009, and was designed to meet specific United States Army and USSOCOM Precision Sniper Rifle requirements. The MSR initially won the PSR competition, and was called the Remington Mk 21 Precision Sniper Rifle in U.S. military service. However, it was then decided that the Mk 21 did not conform to SOCOM requirements at the time in 2018, and the program was re-competed with the Barrett MRAD selected in 2019 as the Mk 22 Advanced Sniper Rifle solution.

The Precision Sniper Rifle (PSR) was a program by United States Special Operations Command to replace all bolt-action sniper rifles in use by United States special operations snipers with a single bolt-action rifle chambered for a large caliber Magnum round such as .300 Win Mag and .338 Lapua Magnum. The solicitation was placed on January 15, 2009. The contract was awarded to the Remington Modular Sniper Rifle and was designated the Mk 21 Precision Sniper Rifle.