A gun that is built to spec will have the link pin staked in place, so it can't fall out. More "modern" guns may rely on a tighter fit of pin to hole, and that should also prevent the pin from falling out.
There's no way to know, without measuring, if the the slide stop is correctly dimensioned, but even if it is, the McCormick mag is essentially out of spec, since it's not of the original G.I. design. It's much more likely that the follower is slipping past the slide stop, than the spring being too weak to lift the slide stop; if the problem is the former, the mag won't drop free when you hit the mag release.
Here's a tip: Don't put your finger on the trigger while the gun is still pointed at the ground. I read that somewhere.
I would have figured that the Springfield MC Operator would have been built right, considering how much he paid for it.
Of course, that is another reason that I dislike 1911's. They are like a box of chocolates.
I had heard that one should avoid the 8 round magazines from any manufacturer. They drop free fine, the just won't left the slide stop.
Being used the the Glock, I put my finger on the trigger right as the gun got to eye level. It was pointed at the bottom of the target because of that silly 1911 grip angle.
I "went off" before I got a good view of the front sight, pretty much as soon as I flicked off the safety.
I left my finger completely off the trigger until I saw the front sight after that.