So as we all know Deimos is one of the worst attackers on Console and barely half decent on PC, and I believe the main reason for this is his lack of fragging ability. I understand Ubisoft not wanting to give him a strong primary in fear of being too strong, but his ability has not been as strong as they anticipated, and the AK-74s fire rate can't even compete with defender smgs. Here are a few options I believe won't be too strong but will definitely help with his fragging potential:

Nomad and Iana's ARX 

With its good damage and recoil yet still being balanced with its fire rate and low magazine, I believe the ARX will encourage slow and calculated play style, maximizing Deimos' effectiveness by having him stay alive and use scans for Intel as opposed to trying to gun the other team down

Hibana's Type 89

A fan favorite, we've yet to see this on another operator. This guns high fire rate and low magazine will compliment a slow playstyle and discourage trying to take multiple gunfights without stopping to reload, while still being strong enough to win more 1 on 1 gunfights

Sledge and Thatcher's L85A2

The definition of an average gun. Solid gun all around. The Sprite of r6 assault rifles. Nothing over powered about this gun yet still no real flaws. Definitely an upgrade from his current primary and still doesn't make him a dedicated gunner.