I view the RNG system like the NBA lottery (and i don't mean rigged lol). It is a random draw, however, certain guns have more "lottery balls" in the basket than others. Example would be: say there are 12 balls in play. 6 balls would be labeled for No Land Beyond, 3 would be for Ice Breaker, 2 would be for Thunderlord, and 1 would be for Gally. Every drop you get has this same breakdown. Thusly, it is more likely to get the lower tiered gun (higher number of lottery balls) every time.