Pretending that there's no acceptable interpretation of this rule, lets parse it;
"Competitor must be permitted [...] to shoot targets in an "as and when visible" basis.
Normally, the and would mean that both elements are independently required - in other words, the rule could be further parsed as;
"Competitor MUST be permitted [...] to shoot targets in an [...] visible basis." I.e. The competitor must be able to shoot any targets that are presented.
The alternative is that "as and when visible" is terribly language written by non-lawyers meant to mean "as they become visible" which seems to be the accepted interpretation. The language should probably be update to reflect what is considered the intent, but hey, Nats, amirite?