44 man has pointed you to the right place. Not to steal 1911 Tuner's thunder, I thought I might synopsize his analysis here, and add a couple of observations.
1911 Tuner points out that as the gun recoils, the rounds in the magazine don't want to move back with it. Instead, inertia makes them try to stay put. All but the top round get pushed back on the nose by the front of the recoiling magazine housing. What pushes the top round back is mainly like-metal friction with the round below it, augmented by tension from the magazine spring.
The last round is treated differently from the other children. The follower isn't like-metal, and the spring is at its weakest for the end of the stack. So, during recoil the last rounds inertia may succeed in keeping it in place well enough to pop it out from under the magazine lips. This leaves it cocked up at an angle where the bottom of the breechface in the returning slide can't pick up its rim and push it into the chamber.
To make up for this tendency, Browning put a dimple on the magazine follower to catch the extraction groove of the last round and prevent it from popping fully forward without help from the slide and recoil spring. If the magazine spring is weak, or the dimple isn't tall enough or is too rounded, the cartridge gets loose and pops up in the way of the slide anyhow.
This problem has been so frequently reported with the SA 1911A1's, that one must suspect either their follower dimples are improperly shaped, incorrectly located, or their magazine springs are too weak. I have a number of magazines with no dimple in the follower. These mostly have rounded followers that try to mimic the shape of a non-existent 8th round. I have no trouble with them shooting reduced target loads in my Goldcup, but they aren't always last-round reliable in my ball gun.
The usual solutions are: new springs or a better shaped follower. I used to buy Mag-pack 8 round conversion kits. These came with a spring and a concave plastic follower with a ball bearing inserted to replace the dimple. I don't know if they are still around? I haven't needed a new one for a decade.
My own experience with the SA magazines, both in the 1911 and the M1A is their Parkerized finish is rough and I can feel the followers drag in the magazine housings. This doesn't help the spring chase the last round to the top. I disassemble them, clean them, polish the sides of the follower, then soak them, and the spring and the housing in Sprinco Plate+ for three days to get some permanent lubrication. More recently I have been experimenting with Shooters Solutions MolyFusion oil for the permanent lube, and it works well. It lets you treat the sides of the follower without lubricating the top or the dimple. I have yet to try the boric acid oils as permenant lubes in this application.
Nick
