So, thinking back to my playthroughs of Fallout 2, and a check on the Fallout Wiki, I found that Marcus has four different items in his inventory when he joins the Chosen One.
CZ53 Personal Minigun x 1
Flare x 1
Antidote x 2
5mm Round Jacketed Hollow-Point x 120
So I just gave him some Fallout: New Vegas equivalents.
Minigun x 1
Dynamite x 1
Antivenom x 2
5mm Round, Hollow Point x 120
From the tests, Marcus should equip the Minigun just fine. But Super Mutant limitations mean he won't actually use the Dynamite. It'll still be available for pickpocketing or corpse-looting, though.
I was tempted to give Marcus a unique Minigun, but I decided this should just be a simple fix, rather than an overhaul.
--- Conflicts ---
It'll conflict with any mod that edits Marcus himself.
--- Bugs ---
None to my knowledge. But tell me if you find anything. I had a few issues during testing of getting Marcus to actually equip the Minigun in his inventory, so some feedback on that would be nice.
--- Intallation ---
Just download and extract the file to your Data folder, then tick it in the Fallout Launcher or New Vegas Mod Manager.
--- Un-Install ---
Just untick it, then delete from your Data folder.
--- Permissions ---
This really only took me 5 minutes in FNVEdit. It uses only vanilla items, so... anyone can feel free to use it or re-vamp it, no permissions or credit necessary.
--- Credits ---
Black Isle - For creating the Fallout series.
Bethesda - For breathing new life into the series.
Obsidian - For creating Fallout: New Vegas.
FNVEdit - The modding tool I used to make this.
Me - I really didn't do much, though...