Okay, I think I understand now.
Combat shotgun, for instance. 5 shots, 1 pump. If totally empty, and you hit Full Reload, (which is "...The amount of time taken is 0.2s plus the time it would take to normally reload the amount of missing ammo.", I just now realized this means PER SHELL), should take 5.2s, but only counts as one action. During that 5.2s, all manners of beasties could crawl up and bash your brains in. But, as you mentioned, if you were Invul, this would be better because you only lose 1 duration of Invul as opposed to 6 (for a reloading 5 times and pumping 1).
However, still with Combat shotgun, if something is rushing you down, and you were totally empty, you could just reload 1 shell (1s), and pump (0.2) to get ready, but this would cost you 2 durations of Invul. Or, as would likely be the case, you're not invul and you really need to shoot things ASAP.
Is that about right? I had no idea actions and time were counted separately until now.
I view it like this:
1) Full/double reloads are worthless and shouldn't be used. Don't ever get used to it. Newbie players should remember that.
2) When invulnerable, ignore the first rule. This is an advanced technique - something you should keep in mind, but no biggie if you forget.
thelaptop noted that dual reload can but used on dualgunner builds for conveniece - use standard reload first and then dual reload becomes esentially "reload pistol in off hand". Fair enough - that's another exception. I don't do that myself, but mainly since I love the sound when switching weapons. Also, reloading, switching, reloading somehow seems more badass to me =)
(note that with dualgunner switching pistols is a free action)