The Gunrunning business allows you to manufacture and sell weapons for passive profits of up to $78K per hour.
Gunrunning is a passive business in GTA Online that allows you to manufacture and sell weapons from an underground bunker. While it can provide a steady income stream, it shouldn't be your primary money-making method. Expect to make between $39K-$78K per hour from Gunrunning, assuming you steal supplies. For maximum profits, continue running crates or vehicle import/export missions alongside your Gunrunning business.
Gunrunning Is A Passive Business
Gunrunning has added a bevy of ways to spend your hard earned GTA$, as well as a new mechanic to earn some more, and the first thing to understand is that a Gunrunning business isn't something you should be pursuing exclusively. It shouldn't be your primary source of in-game cash and isn't a replacement for money making activities like running crates or vehicle import/export.
The most you can expect to make from Gunrunning is between $GTA 39K to $GTA 78K per hour (including expenses). That is assuming that all supplies are stolen instead of paid for too. You can see a comparison between money making methods here.

If you feel you're short of time, switching your Bikers business for Gunrunning is a good choice though. In essence, Gunrunning's gameplay mechanic is most similar to the Bikers businesses, but operates with larger numbers, both in terms of investment and returns. That said, don't expect to get a lot of profit from this business early on, since the best thing to do is immediately spend Gunrunning profits on fast-tracking research.
Buy A Bunker
Before you even get into GTA Online's Gunrunning business, you'll need a bunker. And to be able to buy a bunker, you'll need to be either a VIP, CEO or Biker Club President. Being a VIP requires you to have at least GTA$ 1 million.
Back to the bunkers though... much like in the case of warehouses, they are expensive investments, so you'll want the best value for your cash... which isn't always the most expensive option.
Your bunker will be your base of operations for all Gunrunning related activities and it's from here you'll launch on your Gunrunning missions. There is a garage bay for your Mobile Operations Center (MOC), an HQ section, a weapon workshop and areas for research and production. When buying you'll have the chance to upgrade your bunker with accommodations, a shooting range and a gun locker, as well as choose from a few different styles of interior style.