It's funny: I was an MP in the Marines, and had to carry an M16A2 every day on the job for my first two years in. We had to load magazines at the start of our shifts, and then unload them again at the end of shift. We carried 90 rounds back then before they went to 180 when I was a Corporal. Then, the armorers would issue full mags... for a while. When a spot inspection revealed 8 rounds missing, we went back to ammo blocks and empty mags. However, during that period of magazine-only issue, there was question as to the springs retaining their ability to feed properly when compressed for long periods of time. We knew that our 03 brothers were filling theirs to 28 or 29, but there was nothing in any FMFM that said to do so. Some calls to Quantico and HQMC also yielded negative results on any order, recommendation, or requirement to under-fill our magazines. The Colonel in charge of us (our Provost Marshal) decided that if they were meant to carry thirty, we'd load thirty and be done with it. When it was time to rotate our ammo (which was done about every 12-18 months), we specifically had magazines that were loaded the entire time (for about 16 months or so). They fired as flawlessly as the magazines that never held rounds and were only loaded that day on the range.
How do I know so much about this ammo/magazine stuff? I was the Operations NCO at the time, and I was assigned by our Gunny at the time to make all the calls to find out about the 30-round capacity dilemma. Then, whenever we had to "dispose" of our ammo, we'd go to the Huntington Beach Police Department (CA) range with M60's, M249's, and M16A2's and go to town on bowling pins with the local cops. Ahhh... those were the days!!!
Now, here's the funny part: for my OWN magazines, weapons, etc, I prefer to leave them empty (leaving the springs uncompressed) and the hammers on all my weapons down (again, not stressing the springs). I'm funny like that with my own property, but the point of this post was that there shouldn't be any problem with magazines compressed with thirty rounds. Truth be told, if I had a magazine that had problems with round 30, or with any kind of feeding, that magazine would be segregated for a rebuild/inspection, and if still failing, it'd be junked.
(I was thinking this morning as I was at the range and looking at magazine prices about all the M16 magazines I gave away when I left the Marines. Yes, I am kicking myself for it!!!).
Semper Fi!