A blowback assembly for an airgun includes a valve body and a valve stem disposed within and movable with respect to the valve body. The valve stem has a stepped outer profile and the valve body defines a passageway having a stepped inner profile. The valve stem is normally biased against the valve body to form a seal containing a compressed gas in a valve stem chamber forming part of the passageway of the valve body. When the valve stem is moved from the normal position, compressed gas is expelled from the valve stem chamber to cock the airgun and to fire a projectile from the airgun.