Black key drops plz increase drop rate

I love this game very fun and entertaining plus nightmare mode is fun really fun but I never get a black key drop i had just 1 black key drop out of 100 some games its aggravating how i waste money on highroller and it never works out plz increase drop rates for black keys not extremely but at least a 15% would be nice

just had 6 Nanos from nightmare and still no key

-POiSoN_MeDiC- about SAS: Zombie Assault 4, v1.8