I've been working on my vanguard rank to get a commendation to get the badger ccl, but the only perks I've notices it to have was mulligan and the extra damage on a random bullet perk, I'd rather have better perks and not waste my commendation on shitty perks.
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Only through engrams or faction gifts. The vendor items will always have the same perks when bought from the vendors.
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I believe the only time the perks change are when you randomly get the weapon with a random roll, say out if a nightfall or a vanguard package