A sidearm is essentially "on the side of your arm" it's your backup for your main weapon. So if your main weapon is Shadow Price, then your sidearm is your backup. They're not going to take over anything, the range is atrocious, but fits with how the weapon is supposed to be used, you can easily out shoot a sidearm with a decent shotgun

But how does using it as a primary suddenly stop it from being a pistol? You didn't respond to what he said.