Are you sure all 50 cals are banned there? Over here in QLD only the 50BMG round is discouraged from private ownership - a 50BMG rifle is in the same licence category as a machine gun, but otherwise there is no calibre limit for rifles (I think). I guess at some point it becomes ordanance though.
This is what it says, the Spanish law on weapons:
1 .- considered weapons of war, thus being prohibited its acquisition, possession and use by individuals:
b) Firearms or weapons systems fire size smaller than 20 mm., whose sizes are considered by the Ministry of Defense and war, which are:
"Firearms size equal to or greater than 12.7 mm. ammunition using pod slot in the shorts and flange or rim in the same place."
Firearms using the following ammunition:
Caliber 5.45 x 39.5.
Caliber 5.56 x 45 (or equivalent 223).
7.62 x 39 caliber.
7.62 x 51 Nato caliber.
No weapons of war are considered repeat those weapons that use ammo 308 Winchester bullet type or expanding type ammunition 7.62 x 39 expansive bullet for big game.