1. All switch blades are what is called 'automatic knives'; it's just two different words for the same thing.
2. There are3 types main types of switch blades owners Law enforcement agents, soldiers and all others. Soldiers have their own things, collectors are crazy, so lets talk about policemen, fire fighters and such.
A cop uses a switch blade to send a message, to scare; and that is accomplished mainly by the'clap' sound that a most of switch blades emit while their blade is fired out. It's supposed to scare. Otherwise, a regular combat knife is as scary if not more.
Advice: the bigger the switch blade is, the louder the sound.
Cops also fall, roll on the ground, wrestle and such. You don't want this thing to open in the pocket. A switch blade with a safety lock will prevent that, but he might forget to lock, or hate to fumble to unlock and loose time when facing a danger.
Advice: buy 'flush button' switch blade ( the button does not stick out, it's flush with handle's surface).
All cops also use their switch blades for regular all-purpose activities, cutting ropes, belts or whatever.
Advice: Don't go always for the largest switch blade; it's not pocket friendly, and cops have enough on their belt as it is. Go with a mid-size, around 7.5".
The Boker POLICE-3ZX - 8" open - blade 3.25" - German Switch Blade Automatic Knife.
Fits all our advices and more. Also it's relatively quite affordable.
Have a good one.
Knives at www.menseffects.com