The code that makes a Ranged attack insta-kill is indead in the descr_projectile.txt file. I once made the Armogohma Not insta-kill by modifying that file. Though what I did is modify their Projectile and use it for Trinexx.
Edit:
Having some thing as Tuff or tuffer then a Armogohma. But acts/fights like a chariot. Made Trinexx a little op. So I had to remove the insta-kill quality from him.