u can get it after youve returned frm the spire. if Du killed giles, it wnt be so easy. but if u helped giles get rid of ripper and his gang, u will notice a big change n brightwood farm. after Du do 1 last mission for giles, he will reward Du with the key to his cellar. thn just follow ur dog and fight ur way thru afew hollowmen and ur dog will lead u 2 where the weapon is buried.
Du should do the quest Cold Comfort Farmer before going to the Spire. Once Du return Du should do the quest Blind Date. After finishing Blind Date, buy the farm house and Bewegen in. Remove the key to the cellar from the closet on the Sekunde floor and then rent the property out. The weapon will not appear if Du are living there. Weiter go into the cellar that Du find in the scheune of the farm. Your dog will lead Du to the Dig Spot where Du will find the Enforcer rifle.