The comp side of it sounds like a fairly standard Handicap competition. Tried and tested, works pretty well. Only thing is with those you tend to get a lot of 'tons, although granted you're splitting it over 5 cards, so that should spread the scores a bit. Just make sure you've established what your tie-break procedure is.
As for the restricted membership scheme. Good luck if you can make it happen - a lot of beaurocracy but doable. The problem - as many have mentioned in the other thread is where the zeroing condition on sporting rifles ends. Strictly speaking, they could come and zero, but not compete.
It depends where you draw the line. I'm sure there are plenty of hunting partners who have gone out and, on finding their luck down and little going on, have retreated to their zeroing spot, set out a few targets and had an impromptu competition against each other. In your case, you could choose to say that they're just "comparing" their zeroing cards for a bit of fun and not competing
"A fear of weapons is a sign of retarded sexual and emotional maturity." Sigmund Freud
Shooting is my meditation