The TAR-21 is described as:

IDF’s standard issue rifle, ideal for urban combat.

You’ll find it in central Montoyuc province, in the town of Choza Padre, just south-west of the mountain-top lake in the area. Cartel patrol the area. The weapons case is inside a house on the south-west side of the town.

I approached the house from outside of the town to the west, and decided to just go for it. After taking out the two enemies at ground level, there was one more upstairs, then I secured the area from within the building, taking out the three roaming sicarios with the help of my team mates.

The TAR-21 is a bullpup assault rifle, meaning the magazine loads behind the trigger, and it has great handling and rate of fire stats making it manoeuvre like an SMG in close combat, while offering some of the range advantages of an AR for medium engagements.

The compact SR3M (found in Agua Verde) beats this on both of these stats but has considerably reduced range by comparison.
A 20 round magazine is standard, and both a 30 round extended magazine (available in Mojocoyo) as well as a 50 round large magazine (you can pick up in Libertad) are available for this gun, which both slightly reduce handling but not by much, so they’re well worth seeking out to avoid frequent reloads.