The M40 is a highly modified M700 rifle developed by the United States Marine Corps and used since 1966. Each M40 is produced by armorers at the Quantico USMC base using an original M700 and many aftermarket parts. The original M40 is a Remington Model 40x varmint rifle with a Redfield Accurange scope. It uses the short action and cannot be converted to a longer cartridge than the 7.62x51mm NATO in which it is issued. During its extensive use in the Vietnam wars it proved that civilian rifle was not able to withstand the rigors of combat. Most M40's were converted to the M40A1 in the 1970's to overcome these problems. The main differences being the McMillan fiberglass stock, Hart heavy contour barrel and Unertl scope. In 1996 the M40A3 was introduced, it features a newer McMillan fiberglass stock, a Scheider barrel, a bipod and Schmidt & Bender scope.
The M40A5 designation superseded the M40A3 in 2009, though the evolution between the two systems occurred gradually over a longer period. The primary difference between the M40A5 and the M40A3 is the barrel: The A3's target crown has been replaced with a threaded muzzle and straight tapered barrel to allow the installation of a Surefire muzzle brake and sound suppressor.