I will preface this thought with the following: I am not a lawyer, and I am not that smart or intelligent, so take this with a hefty dose of skepticism; it is also a worst-case scenario.
In some countries, you might end up with something like this:
a prohibition on civilians owning or purchasing new semiautomatics after a certain date
Access to any next-generation semiautomatics would be limited to the military and law enforcement.
They may grandfather previously purchased autos, semi-autos, and any parts needed to maintain those firearms, but they may set a limit on how many of each they can keep in their collection: something like 4–6 are divided between autos, semi-autos, pistols, and rifles; the rest must be surrendered or sold back directly to the feds.
Any new high-capacity magazines with 15 rounds or more will be for military and law enforcement use only, while the sale, importation, and manufacture of new high-capacity magazines for civilian use are banned.
Any high-capacity magazines of 15 rounds or more that are currently owned may be grandfathered in but require a tax stamp or some type of registration mandate.
Thus, putting high-capacity mags in the same column as full autos, suppressors, etc.
That is, all magazines larger than a certain capacity would have to be registered with the federal government, surrendered, or sold directly back to the feds.
They could also mandate that all purchases be paid for with a bank-issued credit card (no cash, not even on the private market).
Also, requiring that a list of those types of purchases be kept separate and periodically turned over to the feds would allow further tracking of the amount of ammunition as well as how many weapons are being purchased.
After all that, one would be limited to what they already owned and what was in the public domain; the only current weapons that one would be able to purchase new are bows, revolvers, shotguns (maybe semi-auto depending on capacity), and bolt-action rifles.
Sorry if my thoughts on this are rambling, incoherent, or even wrong.