Thanks to @IckabobWoW on Twitter for posting this little tip. If you (or a friend) has an Enchanter’s Study building with a follower assigned, you can apply enchanter’s illusions to weapons. The only enchant that it will allow you to apply to ranged weapons is Illusion: Blood Draining. This enchant is actually visible if your weapon is tranmogged to the Wolfslayer Sniper Rifle, as you can see above!
Update: More confirmed guns
Thanks to those who reported more working guns in the comments, Twitter, and reddit!
In addition to these specific guns, their lookalikes (viewable on wowhead) should also work.
Remember, this is only the Blood Draining illusion (but maybe someone awesome from Blizzard is reading this and will open up all illusions to these guns). There have been no bows or crossbows reported to work yet, but if I hear of any I will update the list.
Keep in mind that once this illusion is applied to a weapon it cannot be removed and will always be visible if the weapon is transmogged to one of the supported guns. If you want to see what it looks like, add the illusion to a throwaway weapon and try it out on there.
Update for Patch 6.1: I hate to say it, but Blizzard has decided to nerf this in patch 6.1. I mean, obviously it was a bug, but was it really hurting anything? If you had it previously applied it will stay on that weapon, but whenever you upgrade that weapon it will unfortunately be gone. 🙁