Arc Raiders has plenty of annoying materials to track down for various upgrades, like prickly pears, olives, and mushrooms for Scrappy, but I'd argue none are more annoying than Sentinel Firing Cores. You need to grab four of these pesky Arc items before you can upgrade your Gunsmith to level three, which is quite the challenge.
As obvious as it sounds, it's worth pointing out that you get these Firing Cores from Arc Sentinels, which are sniper turrets attached to ceilings in just a few specific locations, and are different to the regular turrets you commonly encounter. You're less likely to run into these naturally compared to almost all other Arc types.
