It's actually FEEDING that causes the top of the mag to swell.  The rest of the mag is held by the magwell, but the top inch or so is exposed, and when the bolt hits the top round, it creates some lateral pressure that over time will cause the mag to swell at the top rear.  This area of the mag is the weakest part, due to the cut in the rear to allow the follower to engage the bolt stop.

You can use a (padded) vice or C-clamp to gently squeeze the mag body back into shape.

-Troy

I don't know; chances are that the mag body will be worn out before you need to do it more than once or twice.  Mags, *especially* AR mags, are disposable items that will eventually need to be replaced, so having plenty of spares is a good idea.

-Troy