The M4A1 is a standard-issue assault rifle, which can be looted from military containers. It has a 30 round magazine, and can be loaded with 5.56x45mm NATO ammunition. It is accurate out to 1000m, and it is a rare drop. It can take barrel, hanguard, sight, tactical and stock attachments. It is the only gun in the game to fire on both semi-auto and full-auto, but as it uses 5.56x45mm NATO, it's ammo is quite rare.
The M4A1 can, like all ranged weapons, kill instantly with a headshot. It does 50 damage if it hits the target in the chest and does 25 damage on arms or legs. If you are shot with an M4A1 in the arm or leg, you will get the 'Bleeding Out' debuff. If you are hit in the leg with an M4A1, you will have a 'Broken Leg' debuff.
It can be used for breaching, as it does 15 building damage per shot, but due to the rarity of the ammunition, it isn't recommended. It is best used for attack or defense on hostile entities or players due to the amount of damage it does.