You can not buy semiautomatic rifles in Portugal, even if you are a collector I think but for that you might need to have some sort of special requirement.
These are the types of licences we have: http://armas.mai-gov.info/obtencao-de-licencas/
Now you can buy weapons of the same caliber, for hunting (but you need hunting licensing).
Edit: Ok I need to clarify this. Because we have a different name for specific weapons.
Here we can sell semiautomatic rifles (we call them "carabinas", I didn't label them as rifles so my bad), for hunting (a specific class, for boars, deer, etc). The thing is every weapon that's imported in Portugal gets and inspection and a classification if it's a military weapon or not. An AR15 is considered a military grade weapon by our government standards.
So yes, you can buy some specific models of semiautomatic rifles (for hunting), but you can't buy military grade semiautomatic rifles.
You can't buy an AR15.