The M21 EBR is a semi-automatic weapon, unlocked at level 56.
There's actually no such weapon as the M21 EBR in real life - the weapon featured in game is more usually known as the Mark 14 Model Zero Enhanced Battle Rifle, or the M14 EBR.
M21 refers to another sniper weapon system based upon the M14 rifle - and was in fact featured in COD 4. Presumably Infinity Ward used the M21 designation to emphasise this weapon's place in the sniper rifle category.
In any case, the EBR is an improved variant of the venerable US-designed M14 battle rifle, originally designed in 1957. The EBR variant was first seen in 2001.
Like most western battle rifles (such as the G3 and FAL) it fires the NATO standard 7.62 by 51 millimetre rifle cartridge.
In real life the M14 EBR is more of a designated marksman's rifle than a sniper weapon, normally used as part of a squad to provide accurate fire beyond the typical assault rifle's capable range.
This is reflected somewhat in-game, as the M21 EBR deals the lowest effective damage of all of the sniper rifles; Although the base damage is the same, the multipliers mean that this weapon has the smallest one-hit-kill profile.
Unsilenced, with stopping power, a hit to the head, neck or upper chest will kill. Otherwise, the one hit kill zone is reduced - or when silenced without stopping power, eliminated entirely.
However, the EBR also has the lowest recoil, meaning that you can more easily follow up with additional shots if required.
This, coupled with the lower damage, means that most people familiar with the M21 EBR will instinctively fire two shots quickly - if both connect you'll be guaranteed a kill, otherwise depending on your loadout you might get lucky and kill with just one shot.
Magazine capacity is generous for a sniper rifle - it's tied with the Barrett in this regard. The 10 round capacity is more useful with the EBR though, as you can deliver those rounds accurately on a mid-ranged target faster than with the other rifles, courtesy of the lower recoil.
This means the EBR is somewhat of a hybrid, occupying a space between the sniper and assault rifle categories.
The ability to quickly fire this weapon with moderate accuracy lends itself to its middle range capability; Rather than aiming for one-shot kills you should expect to kill in two - a double tap on a target will always kill if both connect.
Unfortunately, the potential rate of fire of the EBR means you will burn through ammunition very quickly - so for your blue perk scavenger can be a viable choice.
Cold Blooded is an effective choice for use with the silenced EBR - you'll be able to more easily evade the enemy and get into position overlooking their rear or flank - in team games you can often eliminate groups of enemies this way before you're discovered.
Lightweight and Sleight of Hand with the ACOG scope is a lot of fun, allowing you to use the M21 EBR in a very aggressive way. This pairing of perks minimises the time it takes to get from sprinting to aiming down the sights, so it's great for situations requiring a quick reaction time.
The M21 EBR doesn't fare so well at long range - the recoil makes hitting a target twice in the far distance rather difficult, often giving your target time to get away.
Like the other sniper rifles, you won't fare too well up close, either - even with the ACOG the EBR is slower to aim than most other weapons, and the sniper rifles have a massive hipfire spread.
Like the other semi-automatic rifles, you can quickly spray in the direction of the enemy and often you'll score a kill - but this isn't a reliable close quarter defence and even if you survive you'll find yourself low on ammunition.
It's best if you can keep your opponent to middle distance engagements - at these ranges you'll find the EBR is a formidable weapon.
It's not a weapon of precision in the way that the other sniper rifles are - but more of a hybrid between the snipers and semi-automatic assault rifles, such as the FAL.
It's a challenging weapon to use at times, and you'll end up in siutations where you would have fared better with a different, more well-rounded weapon. However, the M21 EBR boasts power beyond any mere assault rifle with its potential for one hit kills, and suits more aggressive playstyles than the other options in the sniper rifle category.
Whether you prefer to pick off enemies silently from a hidden position, or to rush with an ACOG attached, the M21 EBR is flexible enough to adapt - and in the right hands is a force to be reckoned with.