Here's an easier way to disarm six enemies without reloading or changing weapons. Do the mission with the Gap Tooth Breach hideout (the one with the treasure hunters) and lay waste to everyone outside the mine.

Then equip a large magazine weapon -- the Mauser is ideal -- and enter the mine. Once inside you basically encounter a trickle of one or two enemies, so you can easily disarm them at your leisure.

This slow trickle also means there is ample time for you to restore your dead-eye meter without fear of being rushed by the enemy.

Another easy way to complete this sharpshooter challenge uses an exploit with the Dead-Eye system (and it could be patched in a future update):

1. You must be far along enough in the game to have aquired the Carcano Rifle and level 3 Dead-Eye.

2. Go to the Twin Rocks gang hideout and eliminate the two snipers on the tall rocks before attempting to disarm the bandits. You must do this because you need to get to the small wall around the building for cover.

3. Go into cover behind the small wall and wait for a bandit to pop up. Immediately go into aiming mode to activate Dead-Eye. Instead of painting the weapons, just put your crosshairs dead on the weapon and fire.

Repeat the same steps over and over until your disarm the six bandits needed for the challenge. Since you are activating Dead-Eye, you will automatically reload, but since you don't manually press the reload button, the game doesn't count it as "reloading a weapon" thus completing the challenge.