Equiping "extra Fusion Rifle ammo" armor does not allow you to hold more ammo in reserve for the Sleeper Simulant (even though we all know it to be a Fusion Rifle).
All year 1 armor seems to grant 2 extra shots though (extra heavy ammo boots from Raids, Prison of Elders, Trials of Osiris as well as exotics like the Armamentarium).
[b]EDIT:[/b] Another Sleeper Simulant bug - It is a Heavy weapon, but it shows up in the Special weapons category in the exotic weapons blueprints.
[b]EDIT 2:[/b] Just noticed last night that the Sleeper Simulant is now showing up under heavy weapons in the exotic weapons blueprints (where it should). They must have just fixed it. Now the only bug is "extra fusion rifle ammo" armor not working.