Question: At the start, why does Tango empty his gun (of unused cartridges) and reload with different bullets before shooting the tanker? Unless I missed something, it's never explained.
Answer: It's not specified, but I would suspect that he changed from a .38 Special to a .357 round or something. You can shoot a .38 Special out of a .357 Magnum gun and maybe for the movie they wanted to add a shot of him doing a reload to a higher power cartridge for the effect. Why anyone with a .357 Magnum gun would routinely carry a .38 Special round is beyond me.
Answer: He was doing a combat reload, where you eject all the rounds, spent or not and fill the revolver with new ones. It guarantees six shots, rather than relying on "indexing" where you count every round fired.
stiiggy