San Juan TX. police seize 1,700 rounds of 50-caliber ammo... two Mexican nationals that were in possession of the ammunition, San Juan police tried pulling over a pickup for a broken taillight. Moments later, officers found hundreds of rounds of ammunition. There were 1,700 rounds of 50-caliber ammunition. The ammo is belt-fed, which means it’s meant to be shot from turret-mounted weapons, much like the heavy artillery mounted on top of military tanks or hummers.