The cow gun is located at a farm just north of Los Romeros in the Vaivenes region. Also shown in Achievement Hunter's video:
For me, the gun didn't appear until after the following achievement:
I got all my 10 kills at a front line before securing that region (I only had Qachas left). The gun also has unlimited ammo, and the damage is purely visual - the enemy will keep shooting at you even when they're cows.