Realistically a streamlining of certificates with a focus on the person rather than the individual guns themselves. This is already the case, and it follows the home office guidance.
A simple certificate or certificates with
1) basic level allowing you to possess and use shotguns with no particular limit to number and or what you can use them for - ie as per Shotgun Certificate.
2) a second level for rifles where you have to demonstrate need. For vermin control and deer management - an initial up to 3 rifles - either rimfires, or small centrefires, or if used on deer, deer legal cartridges. If you need additional rifles then demonstrate need for additional slots - eg for Dangerous Game overseas, or for wild boar.
For target shooting - an initial 1 or 2 rifles for each discipline you engage in. Further rifle slots can be added if you can demonstrate need.
No need for variations provided you do not exceed number specified.
Ammunition Allowance - 200 rounds per rifle you possess in the calibres of those rifles. Amount can be raised on demonstration of need. When you swap your rifle for a different calibre you should change your ammunition.
So for initial 3, you can get a 22lr, a 223, and 6.5cm, but then see the light of day and go into your local RFD and swap the 6.5CM for a 7x57. If you then additional rifles you can get further slots.
Ditto for target shooting, you join a club which shoots traditional service rifles - you add on another slot.
By having such a system there is still tight control over the number of rifles and the amount of ammo in circulation - that is what really concerns the powers that be. There is no public safety benefit from all the paperwork revariation required changing from a 7x57 to a .308.
For new applicants do away with all the bollox of mentorships and land clearances etc. Instead ensure that new applicants are appropriately trained so that they can use rifles safely. I do think there is a big difference between shooting on a range under the supervision of a range officer and be out on your own shooting wild animals. Neither is safer nor more dangerous than the other, they just have very different requirements.
Precedent is already set in Northern Ireland, which arguably post "The Troubles" requires the tightest of gun controls, yet they seem to have the simplest system, a focus of the individual and still allow the possession and use of handguns for target shooting - I will let
@Foxyboy43 and others in Northern Ireland to opine on how well the system works across there.