There are several generations of K100 gun. I had original K100, K100 Mk6 and STI GP6. They are using the same mags. There were at least two different versions of base pads. But the dimensions of mags and pads looks like are the same. I owned K100 for number of years before i got STI GP6 used from my friend and found that mags i already have are difficult to latch in, you need really snap it hard to lock and it will not drop freely ether. The mags which came with the gun apparently had the same issue, but they were fixed by my friend in very simple way - the rectangular latch window on mags was drilled with bigger diameter. So, looks like it is a some dimension change introduced by STI. (Talking about "excellent" STI job - it is not only one problem they introduced in excellent original design - i had myself extractor issues for a while, which i fixed by installing extractor from my older gun. And i got broken hammer parts in my face. My friend also broke hammer twice before selling this gun to me. No wonder that Freedom Ventures now has STI GP6 on sale for $500) Anyway, back to the problem. I did't really like the way my friend fixed a problem by drilling holes in mags. But he claims that they work well after that. Also i was't quite sure that dremmeling the frame of the gun will be be a good option ether. Instead i dremmeled a base pads a bit. And later i found even better solution - i installed rubber covered metal pads from tanfoglio, which i was planing to use anyway - they are heavier, easier to drop and rubber coating helps with mags handling. Several months later i got other gun - GP6C and it has same issue with mags.
It is difficult to tell what are the dimensions STI screwup. Most probably the latch itself, since they changed its design as well. So, the proper solution would be to bring the gun to gunsmith to adjust lenght of mag latch lever, i guess. When magazine is inserted, it should be a bit loose, there should be some space between frame and basepad. To be honest, the magazine latch mechanism and trigger pull are much softer in comparing with original design. The bottom line - you are not only one having this issue and the magazines are not the one to blame. If you really experiencing the same issue like i'm describing, then you should try to insert and drop mag without base pad installed. Even without magazine spring installed, it should fell out just using own weight when you press the button.
Some photos, left to right: mag with original base pad, dremelled out (notice it is slightly thinner) and tanfoglio (it looks thicker but in reality is fits much better)
Untouched and drilled out latch window
GP6C with tanfoglio base pad - notice the gap between base pad and frame
Dremmeled base pad in GP6C - notice the gap
Original base pad in GP6C - notice NO gap
The same original base pad in old K100 Mk6 - notice the gap
The same original base pad in old K100 Mk6 - notice the gap (zoom in)
