Spanish air rifle law allows for (I think) 18 ft lbs versus the 12 of UK law.  A 22 or 25 air gun pellet in the head at 30 metres will kill.  You can buy a gun over the counter and then you should (hm?) register it with the town hall and pay a license fee of some 20 euros a year (what? really? yup!)  Pistols are similarly far more powerful but these are Co2 cartridge guns which are quite expensive.  Ordinary combat guns are pretty ineffective but look the part.  A nice 357 magnum Dirty Harry would certainly frighten me even if it wouldn't kill a mouse.