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.