Other arc raiders items like prickly pear, olives and mushrooms can be annoying, but Sentinel Fire Cores may be even more irritating. It's a challenge to collect four of these annoying Arc items to upgrade your Gunsmith level to three.

 It's important to note that these Firing Cores are obtained from Arc Sentinels. These are sniper-turrets mounted on ceilings at specific locations and are not the usual turrets. These are less common than other Arc types.

 It is a tedious task to track them down and get their cores. There's another way to obtain them, besides the Sentinels. Here's what you need to do.

 How to get Sentinel Firing cores in buy arc raiders coins

 On Dam Battlegrounds, the best places to find Sentinel Firing Cores is on the blue exterior stairway at Research & Development as well as either end of a destroyed dam between Control Tower & Power Generation Complex.

 These are the three most reliable spawn points for these machines. You can move between them in one match, but even this isn't guaranteed. When you kill the Sentinels their chassis will often fall to the ground.

 Sentinels can be found on other maps as well:

 Buried City is located in the courtyard of the Santa Maria Houses and the yellow bell tower at the top of Townhall.

 Spaceport: In Rocket Assembly and Container Storage.

 Blue Gate: Blue Gate is located on the tower and the concrete area to the north of Checkpoint.

 You can also get Sentinel Firing Cores by looting Arc Couriers, which are large Arc containers in the shape of chests that you will find on all maps. They may contain parts for different Arc machines including Sentinels. You can find many potential courier spawns in Research & Development, on Dam Battlegrounds. My advice is to look around and see if you are lucky.