On .63 in general, at NWAF by the ATC, You can't go prone and crawl through the break in the bottom of the walls, that have a concrete ramp on the bottom of it ( concrete slope below break in wall)
I tried all the breaks around the ATC, and the ones that do work are the ones, that have flat terrain beneath them, and no concrete slope (just grass)
It also glitches your characters aim/crosshair alignment, by doing this in 3PP, and somewhat in 1pp, even without pressing the aim button with the weapon.
(Crosshair is center, gun is way below, or above it)