The ARX is a popular assult rifle. The weapon shoots very fast, has a built in laser sight and has reduced recoil for the first few shots of each trigger pull. However, it's high recoil and lower long range damage can make it challenging to use outside of close range combat to the first part of mid range. It's the best assult rifle for close combat, but an SMG will still beat it in that catagory. However, with the right class build the weapon becomes very competative at long range.
Attachments: Akimbo + either Muzzle break or extended clip
Perks: Extra Attachment, Steady Aim
The key attachment you want it the burst fire mode for it. The ARX has recuced recoil for the first few shot of each trigger pull. Burst fire will allow the weapon to settle more often and keep you only firing at the reduced recoil. It also increases the damage and allows the weapon to never be more than a 4 shot kill to the body at long range. The hybrid thermal optic will provide percision long range aiming and can flip out for a mid range zoom as well. The ARX's built in laser sight stack with steady aim and with the 2 hit kill of the burst fire you can rely on hipfire for close range defnese. The grip helps with the recoil at longer ranges and makes the weapon competative at all ranges.
Since you are paying 3 squad points for the extra attachment perk, you might as well double dip. While the burst fire mode conserves ammo consumption, sooner or later you will have to reload. A pair of akimbo pistols with extended clips will give you between 30 and 42 rounds as an emergency back up with a quick press of the y button. Paired with steady aim you can hose players down and don't have to even stop to aim. Beyond here, your equipment and remaining 3 perk points are up to you. There are a lot of options with this build and it's a lot of fun to play.