One Thing you can do is to change the weapon before the ammo is finished. Feels faster for me. Might be Placebo.
The other Thing is to add the cvar cg_weaponcycledelay "0" (standard is 150) in your autoexec or Console.
cg_weaponCycleDelay 150 // delay between accepting a new weapon has been selected
What also works great is to bind the pistol on one of the Mouse Side Buttons.
Hope this helps you out m8