Identifying 20mm round

[spaniard]

Hi guy's. I know this is my first post, but I found this forum whilst trying to identify a shell casing through Google.
I work in a metal recycling plant in southern Spain, quite often I find the odd bit of interesting stuff, Helmets, brasses, knives even a mortar round!
But this one has me stumped, as to what it was that fired it. I know it was manufactured by Lake City in '71 due to the stamps. But what was it made for? AA gun? Aircraft canon?
Any help would be apprieciated.
Heres the pics, I know the others, a 7.92x24.5, and 2 x9mm casings, but whats the big'un?

[zwerge]

Are you sure that`s 20mm? It looks like a .50 cal. spotting round to me.......INERT 50 Cal SPOTTING RIFLE ROUND (OBSOLETE) - SpecialistAuctions.com Rare, vintage and collectables online auctions

[TonyE]

Yes, it is the case for an M48A1 .50 calibre spotting round.

Regards
TonyE

[spaniard]

Thanks to both of you! .50 Cal spotting case !