Well, they don't drop in...and that's why they fix the issue.
Apex identified why the M&P had accuracy issues. Essentially it happens because the barrel becomes unsupported when the slide starts to move. The slide itself actually stretches in response to the pressure buildup inside the gun when a shot is fired and the slide begins moving before the bullet has left the gun. With an improperly fitted barrel it starts to slop around in there, giving unpredictable results. You can check for this by moving the slide just a tiny amount and press up and down on the muzzle end of the barrel and see if it moves. I have a 1.0 that was so bad just applying rearward pressure without moving the slide was enough to make the barrel flop around.
The Apex barrel has more material on the barrel hood, at the muzzle end, and on the fitting pad where the barrel fits into the locking block to allow you to fit those surfaces to the slide and locking block so that the slide and barrel stay in the same relationship to one another until the slide has moved far enough to the rear so that the camming surfaces on the barrel and locking block deliberately move the barrel out of alignment with the slide.
Fitting the muzzle end on a 1.0 can be a bit of a pain, but I did it. It isn't pretty, but it shoots much, much better now. I was holding the black of a B8 with it pretty easily while I was doing a private session with a client.