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/