Because the Mechromancer's "Smaller, Lighter, Faster" skill decreases magazine size, weapons with one-round magazines (such as the Infinity pistol) cannot be used when "Smaller, Lighter, Faster" is enabled.
If you have previously enabled "Smaller, Lighter, Faster" but wish to use weapons with one-round magazines as the Mechromancer, you can disable "Smaller, Lighter, Faster" by using the Respec option at a character customization station.