Why not groups of 3 in the morning with say 3 feeder lanes giving you 84 shooters, then a second round in pairs with feeder lanes as required giving you a further 50 plus shooters followed by a third round in pairs with feeder lanes as required giving a further 50 plus shooters.
Minimum with the above 184 shooter plus the option of feeder lanes in the afternoon, while this may not reduce the overall finish time for the shoot it would certainly reduce the amount of time spent standing on the range and queuing between lanes.
As Shaun has mentioned the BFTA does not stipulate a booking in method so there may be shooters turning up on the day wanting to shoot so potentially even greater numbers
If there are groups of 4 (2+2) in the afternoon I hope there will by toilet facilities close by for the Ladies at least (and plenty of bushes)
If shooters get put off by excessive time spent queuing on the range the problem of over subscription will cure itself but benefit no one.
maybe to late for the first GP but possibly the second one