Modern Magic Rules (PDF)
Next I have an article that goes through ballistics, armor, and weapons for the Modern Magic set.
Modern d20 Campaign Rules
Finally here is the image from the DM guide on firearms:
Firearms Chart
We will certainly be using modern weapons. The future weapons exist but not in starter sets. Those are harder to get a hold of. Explosives as well we can use modern stuff and I will make more futuristic tech for some of this too. Renaissance weapons will still exist, but they are rare collectors items of low power. Also all piercing from ammunition is converted to ballistics damage.
The one tweak I will make to these is that armor and firearms need prices. I will figure out appropriate pricing. I'm essentially converting the worlds currency to the equivalent gold equals GC (Gold Credits), which are the digital version of gold in this world. It's a 1 to 1 to keep things simple in the PHB. I just have to come up with the credit values for these weapons and armor and you can purchase them in game. If people want to swap in guns for their starting character stuff, but want to keep their other starting gear from class or background let me know. We can work something out.
Like I said, this will be a process, but hopefully in the end it's worth it. I know I'm excited to try taking this system to a new place. :)
Just keep in mind all of your characters have grown up mired in technology now. Global networks cover all of Faerun, and even more rural locations have plenty of security and surveillance. A special magic imbues most of the technology as well so that is where the Techromancy stuff comes in from the Modern Magic PDF.
Cosmetic implants, and some cybernetic implants are not completely unheard of, although flashier ones cost far more than smaller utility stuff. There are no hard-fast rules, so feel free to RP it into your characters traits and abilities from classes or races.
Finally, still roll your normal stuff here. For characters either place standard array, then for each trait roll 4d6, drop lowest, if that value is higher than the standard array amount, go with the new value, otherwise just point buy. Other methods may be acceptable, just let me know if you have a preferred method for rolling up stats.