MTCR, ITAR, EAR, NFA ...

The US State Department will throw everything at you if you dare tried to build or possess your own guided guns and rockets, even if you were not from or in the US - because there was a US ally that exported a near-orbital solid motor sounding rocket to a Balkan nation with full factory setup and manufacturing training services without asking anybody in particular, which resulted in the recipient nation achieving prototype production and successful live fire tests of an indigenous surface to air missile system comparable to Soviet S-75 strategic SAM that was eventually selected.

If it weren't for those pile of laws and leverages... building a SAM was doable since 1950s.