AR bolt locks open, last round sits on top of magazine - why?

[IMtheNRA]

Had a strange malfunction a few times today at the range. As I neared the end of the string of fire, the second to last round fired and ejected normally. The bolt locked open just like the rifle ran out of ammo, but the last round is still there, out of the magazine and sitting on top of the follower.

This is an old magazine, Colt brand, UI-manufactured, alloy follower. Other than finish, the magazine is in perfect shape with no visible damage.

Any idea where to look for problems?

 

[dfariswheel]

The magazine is defective.

Possibly the spring is aging and has lost tension.
What's happening is that as the mag runs empty the spring no longer has the power to put the correct tension on the rounds.
As the next to last round is extracted and ejected the weak spring tension allows vibration to cause the last round to pop up out of the magazine.
The magazine activates the bolt latch and the bolt locks open, last round laying on top of the magazine follower.

You sometimes see this in automatic pistols with defective magazines.
The slide locks open and there's an unfired round laying on the magazine, only pointing backward, bullet to the rear.
The magazine is allowing the last round to pop out, and the round flips over facing to the rear as the slide locks back.

Replace the magazine, or the magazine spring.

 

[IMtheNRA]

Thank you. The weak spring hypothesis seems to be confirmed by the fact that when locked open with this magazine, the bolt sometimes slams forward when I set the rifle in the upright rack. Seems that the mag spring is weak and does not completely activate the bolt hold-open.

By the way, do you know the acceptable range of the 20-rd aluminum mag feed lips distance? I may as well check that too.