I thought I'd post this here as this helped
Super easy solution:
Load up level 09 Exodus
Walk 10 meters forward, a little to the right, there's a FY71 on the ground, as well as incendiary ammo. Take both, hold the BACK button, press A,X,Y,B and RB once each, and there you go
This gun had all the attachments and you simply use them all at least once.
Thanks to venialrapier5 for posting the location.