I've been scourging the South-Spanish deserts and mountains for a month now, and was lucky enough (a tip from Richard from Tucotours!) to find a few antique bullets on the Good, the bad & the Ugly opening scene location. (Where tuco jumps through the glass).
They are all used blanks, except one which does not have a triggermark.
I found mostly regular pistol sized bullets (.38 and 9mm), but also one 320 type.
The markings are as follows :
.38 SPL, SB AF
I know for a fact that the Colt 1851 navy edition that Tuco uses in that scene is available in a converted 9mm version. I do not know what weapons the others use, and have no idea where the 320 rounds (Only one, while I have a hand full of the others - took me 5 hrs of searching though) were used for.
It is feasable that they tried various scenes and options, one might have included a small rifle which they dismissed on the set after a few tries, or ditched in the editing room.
Afaik this site has only been used by the GBU opening, not by other films or commercials. It's very easy to recognise the surroundings.
Fun detail : The bullets were always arranged in packs of 5 or 6 within a 40 cm. Probably when they emptied the pistol.
Curious of anyone here can shine some light on the marks and types of bullets!