Garry, with respect, please stop posting nonsense on a subject that you appear to have no practical experience of. Whilst you might have done plenty of armchair research into firearms, your practical knowledge is somewhat limited.
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And we are talking handguns here, small lightweights semi autos or revolvers that can be carried in a holster all shift (along with all the other equipment) need to be small calibre, otherwise the kick from firing the first shot makes it impossible to shoot accurately with the second etc., and small calibre weapons have very poor stopping power.
No police force I can think of uses small calibre handguns (normally categorised as 6.35mm (.25") or 7.65mm (.32"), the common police handgun is chambered in either 9mm or .40", and less commonly .45" (which has called out of favour because it is a big heavy round that can can cause unreasonable injury in a law enforcement role). All of these calibres are easy to carry, size and weight wise.
The alternative deadly force weapons just aren't practical - shotguns, rifles, MP5s, and again carry risk of 'collateral damage' to innocent people.
Again, your lack of understanding shows, the MP5 is chambered for the handgun 9mm round, it just adds a longer barrel for more accurate shooting at longer distance, and a higher capacity magazine.
As for automatics, it doesn't matter how well maintained they are, if the slide spring breaks they become a priest, not a gun. And if they aren't held correctly they can jam.
The way a police officer holds his or her firearm would be handled in the first training sessions they have, or do you think it's like Hot Fuzz and the village Bobby just walks in and grabs a gun without any training?
I worked as an armed officer for 15 years, and can't remember ever seeing a slide spring break, (or any internal spring) have you seen the gauge of steel used in the recoil spring of a semi automatic?
With regards to maintenance, the British Army is still using the Browning Hi Power 9mm pistol in Afghanistan, some of the pistols are 1950's and 1960's vintage, and they still function properly, even in the sand, dust and heat.
The hammer is only safe down on an empty cylinder, when there is a live round there, accidentally catching the hammer can result in an accidental discharge. It has happened many times, especially as the gun is being removed from a holster.
You've been watching too many cowboy movies, while this was true of the revolver carried by Billy the Kid, any modern double action revolver will not do as you say, there is a safety mechanism built into the hammer and it will not fall back into the firing position unless the trigger is pulled.
Also your "hammer on an empty chamber" has another flaw, the hammer is actually attached to the cylnder (you are using the wrong terminology, the cylinder is the round part in the revolver with 6 chambers for cartridges), when the hammer is cocked back the cylinder rotates one chamber, so in your scenario the hammer would fall (if it was possible) on the loaded chamber next to the empty chamber you had left directly under the hammer.
I've had the misfortune of seeing a colleague killed by one round of 9mm to the head, so your nonsense of "3 shots to the head to kill" is just that, nonsense probably based on Hollywood movies.