With a Precision Frame and 450 RPM, the Ammit AR2 is a Solar Auto Rifle in Destiny 2. A solid, all-round weapon introduced with The Witch Queen campaign, getting the AR2 is quite straightforward, and I'll tell you how below.
Where to unlock the Ammit AR2 in Destiny 2 (Location)
You can obtain the Ammit AR2 by heading to the Mars Enclave in Savathun's Throne World, where you can interact with the Relic conduit and take on the Foundry Resonance quest (which gives you two Deepsight Resonance patterns).
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You will need to have completed the Reshaping the Enigma quest before doing any of this, both of which come quite early in The Witch Queen campaign, so the chances you've already done either of the quests are high.
The Ammit AR2 is also a world loot pool weapon, so you have a chance to obtain it by doing your day-to-day activities and opening engrams. Banshee-44 also has a chance to have the auto rifle in his rotation, so be sure to check him out as well.
For more on Destiny 2, be sure to check out What is Cabbage Error in Destiny 2, and how do you fix it? and Is Cayde-6 coming back in Destiny 2 Final Shape? here on Pro Game Guides.