I'm jealous...All of my carry and home defense mags are loaded to capacity all of the time. My range mags are all loaded with five rounds of FMJ as that is the groups I usually shoot. About every three or four months I empty all of the JHP in my carry/defense mags in the,"natural way", at the range. It does no harm whatsoever to leave mags fully loaded. The mag springs wear out from repeated cycling ie; compression and extension cycles not from being compressed. (much like bending a coat hanger wire back and forth until it breaks).
PS: I also rotate my magazines between carry and range mags so that I know that they all function properly whether loaded to capacity or with a partial load. I have all of the mags numbered so that I know the sequence of rotation.
I keep my go to gun's mag loaded and the rest unloaded to save on the springs.
Springs can and do change over time. Cycling them -- especially beyond their design limits -- is what makes the most difference. Storing a spring in a relaxed position will affect it the least.
I store my pistol mags "ready to go" with one less round than they will hold. The last round is hard to get in there, and it was shown in one of the torture tests I read that the magazine springs lasted somewhat longer when doing this. (But this was after like 8k to 10k rounds had been run through them.) YMMV. As already mentioned it's easy to replace them.
So.. are there any downfalls to always having the mags loaded? I had a friend of mine mention that it wears the springs? Is this true? I have three mags for the AK and the Glock always loaded, ready to go. I think it is kind of silly to keep a mag loaded, kind of like a door with no knob if you ask me.
Just to add to this, I've had several Pmag loaded since shortly after Pmags first came out... they were my "go-to gun mags" and I haven't touched them except to occasionally fire off a few rounds (once a year or so) to make sure they're still functioning. That is without the feed-lip protectors/mag covers on them.A further note:
Depending on the magazine construction, leaving them fully loaded for long periods of time CAN cause issues with the mag body...
The rounds in a magazine exert pressure on all sides of the mag; the majority of which are the "sides" of said magazine. A fully loaded polymer-only mag can, over time, swell outwards under the pressure of the rounds on the inside.
If the feed lips are unreinforced polymer, these, too, can deform under the upward pressure.
Of course, this may or may not affect feeding...
My recommendation for 100% polymer mags (aside from the spring) is to download by a few rounds to help alleviate the stresses on the mag body and the feed lips.
-J-