This method here can better your chances. Im not going to say its 100% guaranteed , but I have seen it work multiple times. Equip yourself with everything solar even your subclass. Use a rocket launcher a lot during your raid or nightfall, or what ever you are playing. I noticed a certain trend in the game, and when someone is running around with a certain element a high percentage of the time a weapon of that same element drops. I also noticed when you use a certain weapon type a lot that weapon tends to drop a lot for you. I was denied the Black hammer for a while, and noticed all my clan mates that snipe alot received the weapon. I then starting doing the same and put everything solar, and bam I got the Black hammer. The idea hit me when a friend kept getting every weapon pretty much void. He was a warlock and always ran with Void walker, and pretty much used void weapons. I had a friend who was in search for the Gjallarhorn. I presented him with this idea, and mind you he was playing his ass off for months trying to get the Gjallarhorn. He did what I suggested and in his 3rd raid the Gjallarhorn dropped for him. Im seeing it drop a lot in the Crota raid. Give it a shot and Good Luck