Why the change?

[titoli]

I always wondered (by the second time I saw the movie, if I remember well) why Mortimer changes from rifle to revolver to kill Callaway.

[Johny_Exhale]

he probably has a better aim with it, and he didnt expect to shoot long distance

[cigar joe]

The rifle brings down the horse, then as Calloway gets closer evidenced by the pistol shots that reach Mortimers feet then Mortimer knows he's in range for his pistol just a preference perhaps.

[drinkanddestroy]

maybe it's part of the whole Western convention of "fair fights," ie. you can't fight someone with a pistol if you have a rifle and he is out of your range; so he uses the rifle to shoot the horse and force Calloway into the "duel," then it's a fair fight, pistol vs. pistol. That could also be why he waits till Calloway is within Calloway's pistol range before killing Calloway, all part of the "fair fight" convention, eh? 

[stanton]

But he doesn't give him a chance. He shoots him before he reaches the range of his pistol.

And that's the idea of the scene. Mortimer's supremacy due to his superior weapon arsenal.

[Faisalha9159]

I think it was a way of showing that he was a professional in every way and a very different character from Manco despite being in the same profession. He had all the tools of the trade and also knew when to use which tool, and how meticulous he was in the execution of his ar(excuse the pun). Very evident when he attaches the extension butt on his pistol for longer range and also in the way he placed one solitary bullet in a very precise manner right in between his preys eyes.

[Cusser]

I think it was a way of showing that he was a professional in every way and a very different character from Manco despite being in the same profession. He had all the tools of the trade and also knew when to use which tool, and how meticulous he was in the execution of his ar(excuse the pun). Very evident when he attaches the extension butt on his pistol for longer range and also in the way he placed one solitary bullet in a very precise manner right in between his preys eyes.

Agree

[Dirty Rat]

It also showed to the audience that he was cool as f@ck!

[stanton]

It also showed to the audience that he was cool as f@ck!

Which surely was the main reason ...

Doesn't make much sense too check Leone's western too much for logic and "reality".