The M21 is a marksman rifle adaptation of the M14, designed in 1969. The United States Military chose the M14 for its reliability, accuracy, and the power of the 7.62mm cartridge to become its primary marksman rifle following the Vietnam war. The modification was achieved by simply attaching a 3-9x variable zoom optic and by using National Match-grade ammunition with the weapon. It was the U.S. Army's primary marksman weapon until 1988, when it was officially replaced by the M24, but still saw some use during the Iraq War. Additionally, a newer version of the rifle, the M25 is used by some special forces units.
In Battlefield Vietnam, the M21 is an option for the ARVN and US ArmyScout kits. It has a 10-round magazine, high recoil, high damage, high accuracy and a medium rate of fire. It can kill in 2-3 shots, or a single head shot. It is equipped with a 6X scope meant for long range engagements. It can be considered an equivalent to the NVASVD.
When using the M21, one should go for sniping at long ranges, because, despite its small cross hairs, hip firing can be a wasteful and ineffective, even in close quarters. When sniping, naturally, aiming for the head is recommended, for easier kills and especially conserving ammo, as the M21 only comes with 2 spare magazines. Generally, manually reloading with the M21 in unadvised unless there is very few rounds left in the current magazine. A good way for ammo conservation would also to be set up near an Ammunition Box.
Because Battlefield Vietnam features different loadouts for each faction, this list may not be specific enough. Please see each faction's respective page for information on individual weapon kits and loadouts.
"The U.S. Army needed a sniper rifle in Vietnam and refitted high quality M14s with scope and match grade ammunition. Designated M21, this remained the primary sniper rifle until 1990."
— In-game description
In Battlefield: Bad Company 2: Vietnam, the M21 is a sniper rifle issued to the Recon kit. The M21 is semi-automatic and has a 10-round magazine. It uses a sniper scope, with a notably lower zoom that the other sniper rifles', and it allows for some peripheral vision, comparatively similar to the 4X Rifle Scope. It is notably identical to the SVD in stats. Because of the weapon's low powered scope and good peripheral vision it is much more suited for close to medium range combat than the M40 or SVD.
The M21's in-game description and stats evaluation.