[yarddog]

The hole you speak of serves two purposes-one is to lighten the weapon, the other is to lighten the slide so that the "timing" is right to function the weapon properly. The longer slide is heavier, and the weight needed to be reduced for functioning

That is probably part of the reason but every other 17L I have seen has had a ported barrel in which the ports open right where the opening in the slide is.

[5Madman2]

The ports are a different story than the Long slide-the ports need the opening to vent the gasses.

The long slides (34/5 I believe)need the weight reduction