Ok, I did the trick on the 360 and now have the unlimited ammo mutator. I wanted to see if it would be possible to do this on the XB1, but from navigating the memory settings on the BC version, I can tell it's not possible without the 360 version. I can do a quick write-up on how to get it if you guys want additional coverage. If you already have turrets opened up in Horde mode then I think you're looking at 15-30 minutes of grinding max. If you don't have turrets, it's going to take you about 30 minutes to unlock turrets and then another 15-30 to get all the medals.