The G28 sniper rifle is described as:

A modern, German semi-auto sniper rifle.

You can pick this up in northern San Mateo province deep inside a cartel prison in a deep deep canyon. The “La Carcel Del Pueblo” mission is nearby. The weapons case is in the armoury at the back of the base if you approach from the main entrance from the north.

This base is very heavily populated, and once you get through the main gate and past the first base you go up a gully in the canyon, and there’s the main base. Unintentionally, I ended up attracting attention, but it worked well to funnel enemies into the choke point in the canyon and take them out from behind cover.

The map shows another road into the base from the west which takes you in much nearer the armoury and is possibly an easier approach.

The G28 is a good, accurate, long range sniper rifle, and can fire semi-auto as fast as you can pull the trigger.

The standard magazine is a decent 10 rounds, with a 20 round extended magazine option.