The Mosin ammunition (lose stacks and ammunition boxes) only spawns at military or industrial/shed buildings nearby military structures.

This is shown at four military areas:

Myshkino military tents

These buildings spawn the ammo: http://img.dayzdb.com/img/buildings/land_guardshed.jpg

NWAF (Northwest Airfield) military tents

These buildings spawn the ammo: http://img.dayzdb.com/img/buildings/land_guardshed.jpg

The blue warehouse buildings beside the airfield spawn ammo: http://vignette2.wikia.nocookie.net/dayz-origins/images/d/d9/Land_Hangar_2.jpg/revision/latest?cb=20151127041637

Kamensk military base

The two industrial buildings to the west directly of the barracks, coordinates (077, 005)

Pavlovo military base

Same occurs in the brown sheds nearby barracks