Dan Helmick is a d20 modern fan.
In this article there are some tweaks to adapt d20 Modern Core Rulebook to 5e rules: http://dnd.wizards.com/articles/features/5e-modern-firearms
Rules for 5e modern:
- Light Armor – Heavy coat, Leather jacket, Kevlar-lined coat, Undercover vest
- Medium Armor – Concealable vest, Light-duty vest, Tactical vest
- Heavy Armor – Land Warrior armor, Forced entry unit
- New Damage type: Ballistic damage
- Aim Action
- Firearm Proficiencies by Class
If you are looking for the official d20 Modern System Reference Document you can find All-in-One MSRD Download here:
http://www.wizards.com/d20/files/msrd/MSRD.zip
Dan is also answering on d20Modern here: http://www.reddit.com/r/dndnext/comments/32gt63/5e_modern_firearms/