Rifle Associations can surely play more important role independently of Arms Act 1959. Why they are not doing it? Because many of them have been practically taken over by corrupt and vested interests. Linking such organization legally with the Arms Act 1959 like making a legal precondition of "certificate" etc. from these organizations for arms license would create more problems for arms license applicants.
As far as question of non prohibited bore, prohibited bore and prohibited arms etc. all these are unconstitutional creations and figments of imagination put into the Arms Act 1959. When arms are fundamental right under the Constitution, then how can any arm be prohibited in contravention to the Constitution? State may create more legal paperwork, higher checks and balances etc. for certain type of arms but cannot blindly "prohibit" them. I think you are able to get my point.
All things are subject to interpretation whichever interpretation prevails at a given time is a function of power and not truth. - Friedrich Nietzsche