@SeaToSki
Machine guns are fully automatic, so they can reel of hundreds of bullets without pausing or the trigger having to be pulled for each bullet. They are mostly mounted to vehicles to carry the ammunition or take two people to operate, one to hold the gun and one to carry and feed the belts of ammunition.
Semi automatic guns are the next level down and can fire bursts of bullets with one trigger pull. Most police forces and many armed forces carry these guns as they are more portable because you dont need a box of ammunition for each time you fire. They have large capacity magazines and the operator carries multiple refills in their pockets or strapped to a vest thingy
Then there are ordinary guns which require a trigger pull for each bullet fired but can have a magazine with multiple bullets
Then there are bolt action guns (mostly rifles) which require each bullet to be loaded and fired individually
I hope I got that mostly correct!
Good effort but lots of it is wrong I'm afraid.
A machine gun is belt fed and fires fully automatic for as long as the trigger is held down. If you are skilled then single shots are possible though 2-3 round bursts are the norm, both for accuracy and to avoid overheating. This is not a police firearm in the uk. Regularly mounted on military vehicles but also carried by individual soldiers at section level for fire support.
Semi automatic means the trigger must be pulled for every shot, firing one round only. These will be magazine fed, typically 20 or 30 shots in a rifle, less in a pistol.
Sub machine guns can fire fully automatic or on semi automatic but are smaller firearms and fire smaller ammunition, typically 9mm rounds. Short range typically.
For many years and many roles the heckler and Koch mp5, a submachine gun, has been the preferred police weapon. This is what you mostly see in airports and is what the was used in the Iranian embassy siege in the eighties. However, UK police use semi automatic carbine versions of it.
If it's more assault rifle in look it's likely to be a heckler and Koch g36, a sig sauer sg516 or a sig sauer mcx carbine (that's what they had when we had our local hostage situation a couple of years back). These are semi automatic duties firing the same calibre as NATO military assault rifles (5.56mm). They are extremely sexy looking 🤓
Bolt action rifles have a small magazine which feeds a fresh round with each manual operation of the bolt and are single shot. More a sniper weapon now. My grandfather and great grandfather used these in the first and second world wars with the Lee Enfield rifles - although my grandfather's main weapon was a 3.7" anti aircraft gun and his father carried a lance and sword!!!