Hi,
First of all, please excuse me for my poor english, the vocabulary, specially the technical one, is hard to use properly and I don't know much about it. In addition, I'm not sure that I am posting in the good section, if I made a mistake, please correct me and accept my apologies. Anyway, let's try

When using a repeating crossbow and even with a BS of 4, the cumulated malus of: long range, cover, multiple fire and movement give you a "7+" to strike, but what rule should we apply, given that no 7 can be rolled on a regular dice

?
1) The Warhammer and Necromunda rule: on a 7+, you must roll a 6 and then 4+ on an additionnal roll to strike
2) The "a 6 is always good" rule: you always hit on a 6.
3) The "after 6+, no one will ever save you": you cannot fire if a 7 must be rolled.
No need to precise I tried to find an answer by myself, but Google gave me nothing

Best regards,
Ekko