Roc strikes have an estimated 1/100 chance of an exotic drop. There are 23 weapons and around 20 armour pieces this would translate to a drop rate for a specific exotic at around 1/4000 Their are known exotic drops in the raids with consistently higher drop rates but you can receive any of the 50 ish exotic from it so it its a 1/5 chance of an exotic drop from the known exotic drops in the raids your looking at a rate of around 1/250 for a specific item. Night fall has a equal (estimated) drop chance as a raid drop so 1/250 for a specific piece. Heroic weekly has an estimated lower drop rate for exotics at higher than a roc struck but lower than a night fall around 1/500 (estimated) Therefore if you do these things you will have these rough chances at getting one. It does not matter how any times you do it you have the same chance of getting it each time. Tldr: For a specific exotic drop like gjallarhorn. Raids (known exotic drop points.) + nightfall (estimated drop rate of 1/250) Heroic (drop rates estimated at around 1/500 ) Roc strikes (estimated drop rate of 1/1000) Good luck guardian. Pray to rngesus.