The StA-5X Irradiated Petrusite Cannon is one of the latest additions to the Helghast arsenal featured in Killzone 3 and is manufactured by Stahl Arms. It is very powerful, killing a normal soldier in just one or two shots. The Arc Cannon can be charged by holding down the firing button, which causes it to become more powerful while consuming more energy. It is the most advanced of the Helghast weapons usable by footsoldiers, being a massive improvement on the mostly conventional Helghast weapons.
The gun utilizes irradiated Petrusite, which is a highly unstable material taking shape of a fluorescent green gas or plasma. Enemies killed by this weapon will explode in a cloud of bloody mist. Unlike the VC5 Arc Gun, the StA-5x Arc Cannon does not have unlimited ammunition and will require either changing weapons or refilling ammo at ammunition crates which are common in the levels they're found in.
There is no sight on the gun, but it rarely misses due to an unexplained form of auto-targeting and its high blast range. Perhaps this is due to a biological factor where irradiated petrusite is attracted by the weak but existent electrical current produced by all living organisms.
It is first found during the mission "Stahl Arms Infiltration", where it can be very useful against the huge waves of Helghast fought in the labs. It is also seen being used by the Helghast in a cut scene during the mission in the jungle.
- Strangely, only Orlock's head explodes when he is killed by Stahl, whereas all others killed by the cannon vaporize entirely.
- Killing 3 Helghast with the StA5X Arc Cannon at once earns the player the "Frazzle Dazzle" trophy.
- It is the weapon being used in the Narville execution cutscene both at the beginning of the game and again midway through.
- In the cutscene where Sev witnesses an ISA prisoner being torn apart by the Arc Cannon, and also in the cutscene where Stahl kills Orlock, the StA5X appears to fire a stream of irradiated petrusite. When it is used in-game, however, it can only be fired in bursts, not in a stream as portrayed in the cutscenes.
- There is a small green energy bar on the weapon just above the ammo counter.
- While the STA5X is a very advanced weapon, it overheats very quickly, showing it is likely still in the prototype stages of production. (Maybe the letter "X" in its name stands for prototype, as real life guns in prototype stage also sometimes have "X" in its name, e.g. the well-known XM-8 assault rifle, which was never developed into serial produced version.)