The Acrid is a Grineer weapon unlockable through Bio Lab Research in the dojo. It is a toxic weapon system that delivers toxin-filled flechettes, rather similar to the rounds from the Boltor. Unlike the Boltor rounds, the Acrid's rounds are not susceptible to gravity.
As of Update 10.3, the toxin mixture in the barrel is affected by the Acrid's energy color, and the tiny explosion effect on needle impact. Note that the explosion effect is purely cosmetic.
As of Update 11.3, the Toxin effect's DoT scales with Acrid's base damage, not including elemental mods. To clarify further, onlyHornet Strike and Magnum Force directly affect the DoT. Damage per tick is 50% of this base damage (a rank 10 Hornet Strike will yield a base damage of 112, with the DoT dealing 56 Toxin damage per tick).
Oddly, despite its incredibly high base accuracy the Acrid's accuracy value is heavily degraded by Magnum Force well beyond what the mod's penalty would suggest, making the weapon highly inaccurate as a result.
To clarify on how the weapon proc works: the weapon will proc toxin on every single shot, unless another elemental status procs. So if you mod the weapon to deal only corrosive damage using an electricity mod, the weapon will proc toxin on every shot unless the corrosive procs. The proc chance listed on the weapon is actually for any other elemental damage that is created by the weapon.
Semi-automatic firing can be countered by simply adding an alternative way of firing on 'scroll up' or 'scroll down' through the key binding customization.
Before Update 10, Saryn + Mire + Torid + Acrid would give you the only loadout consisting solely of toxin.
As of Update 11.4 the Acrid is physically the largest handgun in Warframe.
One definition for Acrid is "sharp and harsh or unpleasantly pungent in taste or odor", likely owing to the gun's toxin-based damage.
As of Update 9.5, the Acrid uses a different reload animation than other pistols. It consists of disconnecting the barrel for a short period of time and reattaching it to the gun.
The writing near the barrel says "Obey" in Grineer language.