It's not a good idea though to leave a gun loaded and unattended for extended periods.
I agree, its generally not a good idea. But it ranges from being not being a good idea to being a BAD IDEA depending on if, and how the gun is stored, and the security of that storage.
Based on what knowledge or fact?
In my case, it's both knowledge and fact.
Granted, it’s been a while since I’ve unloaded my handguns and taken the time to clean them and re-oil them but there’s no doubt in my mind that any one of those guns that have been sitting there loaded for the last probably year and a half without having nothing done to them will fire when ready so I don’t know where you’re getting this from saying it’s not a good idea to leave a gun loaded and unattended for an extended amount of time.
There are many cases where something that is generally not a good idea may work acceptably well in certain specific situations, but that does not change the fact that they are generally not a good idea.
Loaded, and unattended (and unsecured????) for an "extended amount of time"... what is that, to you? The time I mean. A year? 2? 10? 20? 100??
Mechanically speaking, leaving the chamber(s) loaded is of no concern. Leaving the gun COCKED, could be. And this is where the concern about springs comes from. Older firearm designs which use a flat "leaf" spring should NEVER be left cocked. And that belief was carried over to guns with coil springs, and in the early days, was also just as valid.
Today (and for over the past century) properly made coil springs don't take a set or fail if left compressed for long periods of time (provided the compression is within their design limit). Unfortunately, the only way to know the difference between a spring made properly and one that is not is by when it fails. And, then, it too late.
As a side note, the question about how long a gun could be left loaded, creates the question of "why would you need to leave a gun loaded for a long period of time?"
What scenario do you envision that would require you to have a round in the chamber (and UNATTENDED, which is the biggest issue) for an extended time?? I can't think of any. OR, at least none that apply in my life.
So, as I see it, from the mechanical side, its a non-issue.
BUT,
From the practical side (and this included SAFETY) it's a big issue. A loaded, unattended gun is a risk. ALWAYS. Its not the loaded gun alone that is the risk its the loaded and unattended that is the risk. When you aren't there, you do not, cannot, know, with certainty what the condition of the gun actually is, until you personally examine it.
Loaded, unattended AND unsecured is the triple crown of disaster, and unlike horseracing, foolish people win that one many times, every year.