this might take some grinding to get. i got lucky and got in on my 8th try. after u reach lvl 50 try ghost ship chaos then get to observation deck.. ( clear kitchen area and get the key use elevator and go to casino after getting to main big hall take a left and use elevator once reaching observation deck go to the other side of the room and grab the weapon case). retire and repeat until u get a lvl 50. i got AUG lv50 with 5 slots..
credit goes to Prov Drummer for his great advice.. probably the fastes way
hope this helps everyone who doesnt know how to obtain one
u can try paricipating in residentevil.net online event but takes ages :)
NOTE: rare finders do help alot so use them. try to equip all 3 on all your 3 guns