I've found this 3ºd party spell:
https://www.d20pfsrd.com/magic/3rd-party-spells/adamant-entertainment-3rd-p arty-spells/readied-pistol
And says "target firearm is loaded and can be fired immediately". This means you can make an attack after casting the spell or you have to wait until your next turn?
And, if you have an special bullet loaded, can you make the five shots with that special bullet?
First, this question should be in the Third Party advice and rules questions forum. I've flagged it to be moved.
And says "target firearm is loaded and can be fired immediately". This means you can make an attack after casting the spell or you have to wait until your next turn?
No attack is made as part of the spell, which means you have to spend actions as normal. If you cast the spell as a standard action, you can't use a standard action (or full-attack action) to attack that turn, obviously.
And, if you have an special bullet loaded, can you make the five shots with that special bullet?
No one knows, because it's a badly written third party spell, the text can be interpreted multiple ways (including not adding the bullet's base damage to the shots). I'm pretty sure duplicating special ammunition is not intended - as a GM I'd change the spell to requiring an unloaded firearm as a target.
You may want to look at the Spell Cartridges feat instead. It's also not exactly well written, either, but at least has author commentary (here and here) on how it's supposed to work.