Summary:
1. License: you need to pass the relevant test and apply for a Restricted PAL to be allowed to acquire a restricted gun, e.g. the Remington R-15.
2. Range use only - you are only allowed to fire restricted firearms on a range that is approved for restricted firearms. In practice, most such ranges belong to clubs, so you have to join or be a guest at one of these clubs.
3. Transport - you need a permit called Authority To Transport (ATT), specifying the restricted firearm(s) and which ranges you can take them to.
4. Magazine capacity: as a semi-auto centrefire long gun with a detachable magazine, the magazine for it must not be capable of containing more than five rounds. But as 7.62mm mentioned in post #7, if a magazine that is not for this firearm happens to fit and function with it, it can be used with it even if it has a capacity greater than five rounds, so long as it doesn't run afoul of the regulation by being made for another gun subject to the same limits. E.g. the ones he named are made for a handgun, so they are allowed to be ten rounds, but magazines made for another semi-auto centerfire long gun would also be limited to five rounds.