Say my BS 3 hero wants to shoot a model. Normally a 4+ to hit, no problem. But suppose the target is under cover (5+). And at long range (6+). And my hero moved (7+). Since it's pretty hard to roll a 7 on a six sided die, it looks like the shot's impossible. However, I seem to remember some house rules that allow models to "roll a 7." Are there any house rules for this situation?