Hi yar89
Welcome to the forum.
If you read through the older posts, I think you will find that some of the more experienced engravers have in past times used nails and other materials to use as gravers. That means your drywall screw isn't to far out of line
This post will give you enough information to get started:
Inexpensive Way To Give It A Try
There is another post along the same lines posted by airamp and can be found here:
http://www.handengravingforum.com/showthread.php?t=2365