The Archgun Deployer is something added in the game as part of the Fortuna expansion. As part of the expansion, a new faction with new Standing mechanics was introduced, known as Solaris United. Much like Konzu in Cetus on Earth, there’s another faction to grind Standing with various rewards through Bounties.
As you undertake those bounties you will reveal a variety of different gear items that you an use to make your gameplay more fun and more engaging. The Archgun Deployer in Warframe is one such item. This item will allow the Archgun, which is normally locked to your personal “spacecraft” in Archwing missions, to be used in normal ground-based missions. Basically it’s a form of air support.
The key to getting the Archgun Deployer in Warframe is to finish the Profit Taker Heists. This series of missions is only unlocked once you reach the higher ranks of Standing with Solaris United. You could do this by completing bounties for Eudico at Fortuna.
Once you have the Profit Taker Heists unlocked, they can be found in the back room at Fortuna. The missions aren’t too hard, just speak to Eudico in the hidden room to get started.
You will also need a Gravimag to attach to the Archgun itself. You can get the blueprints for these in the Tenno Research Lab in the Clan Dojo. This will allow you to actually use the Archgun and call in what is essentially an orbital cannon. To use it, you need to equip it to your Gear Wheel, which can be done in your Arsenal on the Orbiter, or through the menus. You will also need to go to the Vehicles tab in the Arsenal, and equip which Archgun you wish to use with it in the Heavy Weapon slot.
You have five minutes, or the capacity of the magazine, with which to fool around with the ArchGun before it’s expended.