For a time ago I surfed around on the net and found a nice PDF-tutorial of how to make new weapons in Photoshop. There was pictures and lots of text. Do you have any link for this kind of tutorial? Because I can't remember where I found it.

http://archive.forums.soldat.pl/topic.php?topic_id=4802
http://www.turrino.it/page.php?id=14&a=dl
I belive that what your looking for. I used Search.