As things stand Open you have approx 3/80 chance of winning something, Recoiling 1/10, .22 1/4, ladies 1/10, juniors 1/3. (These are only approx amounts buts as you can see in Open the odds are against the average shooter).

With grading, you would only need 3 grades and a 1st, 2nd and 3rd in each.

A grade 85-100%
B grade 75-84.9%
C grade lowest-74.9%

This makes shooting PCP or recoiling irrelevant. I suspect B grade would be for average shooters and C for Newbies.