I've seen Gjallarhorns dropping like crazy recently. I got one from this week's Nightfall. It was my 4th. I destroyed it. I have been in fire teams where it has dropped 4-5 times in the last two weeks or so. At least 2-3 times for beating Crota.
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It's probably going to become a lot more common right before the DLC comes out and then there will be something new that makes it basically pointless to have, or you won't be able to upgrade it
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I also got Gjallarhorn in Nightfall and wow its dropping that quick?