The M40 entered Marine Corps service in Vietnam after commanders saw a need for dedicated marksmen against an unconventional foe that already fielded trained snipers. Leftover World War II rifles proved inadequate, prompting a 1966 search limited to commercial options that used standard 7.62mm NATO ammunition. Testers chose a Remington 40X-based design issued as a complete package with a Redfield variable-power scope and match-grade cartridges, the first time the service treated a sniper weapon as an integrated system. Hundreds of the rifles reached the field and delivered reliable hits, though tropical humidity warped wooden stocks and produced rust and other failures. Those shortcomings led to the M40A1, which used a fiberglass stock and stainless barrel and remained the Corps’ primary sniper rifle into the 2000s.