When your fighting for your life you want a reliable gun.
He would have been checking for timing of the pistol.
Yes he may not have done a good check in the time he had but it's how others with more time would do it. .
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My favorite scene with him is when he stumbles into the general store after surviving his ordeal in the desert. He needs a gun, and he needs it now, but he isn’t going to settle for any pistol.
After the shopkeeper shows him his wares, Tuco disassembles various Colt 1851s that had been converted for cartridges, and proceeds to assemble a pistol from their various parts. He inspects the components, checks the timing, and then test fires his build, as the slack-jawed storekeeper witnesses his marksmanship.
It's a great Western. There was an interview with the actor that played Tuco, (also played the bad guy in the Magnificent seven) he said something like he didn't know what to check on a gun in that scene so he listened to the mechanism and the director liked it.