Even if he took the bullet out he would have seen it was indeed a blank. He would have had to take the bullet out and hold the barrel up to the light with the chamber open to see a previous bullet was stuck in the barrel.

Actors’ job isn’t to be as knowledgeable as a firearm consultant, hence why they hire one. The same way they trust any scene has been safely planned out before hand and the giant boulder is assumed to be fake and not a real rock, and the harnesses that suspend them weren’t set up wrong so they fall and break their neck.