The limiting factor for using an AR platform as the base gun for a bullpup build is the receiver extension ("buffer tube") that extends well back behind the action and is required for MOST
ARs to function. (And preclude putting the action back far enough for a bullpup.)
I said MOST
because there are some piston systems and rimfire ARs that do not
require a tube/buffer/spring to operate and might be viable basis for a bullpup build.