Are you god?

The more you think about it, the better it fits. TF Industries Sentries* have to be deployed by hand, but a well-coordinated team of Engies can get a solid wall of supersonic lead in less than a minute. Aperture Science Sentries are more portable and can be mass-produced, but quality control can be turned off with a few button presses

*Engineer's "designed by me" line has been retconned; the in-universe patent is held by TF Industries