The M16A1 is an

American assault rifle, it is the default primary weapon of Adam and Cathy, it is a relatively a good gun, having good range and accuracy, it holds 30 rounds a clip and 90 rounds in reserve.

It is the default weapon of the defensive type characters. Most people playing the aforementioned characters usually ditch this weapon for better weapons, (e.g LWRC, HK 416 & Premium Counterparts, and AK-47). This weapon is actually not very different compared to purchasable counterparts other than high damage rifles like the AK-47.