This article covers a subject that has been deemed non-canon by either the author or the Alien vs. Predator licensees, and thus should not be taken as a part of the "real" Alien vs. Predator universe.
"Alternate ammo type; propellent-assisted rounds exit the pistol's barrel at a significantly higher velocity, causing tremendous damage."
The A27 is a ammunition type which send propellant-assisted rounds that exit the end of pistol's barrel at a significantly higher velocity than the standard ammunition which causing tremendous damage, even powerful enough to penetrate Xenomorphs' mesoskeleton. However it gives the weapon a higher recoil, thus reducing the rate of fire.