I am going to attempt to engrave screw heads on various guns .Just screw heads , nothing else . so I will be cutting short straight lines.
Can I get advice about what number graver and secondly how to sharpen . Thanks in advance

You should go to this site for basic engraving info. It has almost all of your questions and is simple to understand. The main thing is to learn how to sharpen the gravers correctly before you start on gun parts. www.engraverscafe.com

Screw heads are not a "piece of cake" Magnification is so important to make the divisions equal. I speak from experience...Make a simple pattern.. leave the eye poppers to the pros.