Bullpups have advantages over conventional rifles.
They have a shorter overall length than an equivalent rifle with the same barrel length. This makes them handier for infantry (especially mechanised/airbourne units) or troops operating in built up areas where long barrels can be a pain. The US addressed this problem by issuing M4 carbines which have much shorter barrel lengths and hence overall length whilst incidentally reducing performance. When you consider the relative ineffectiveness of 5.56mm it makes you wonder why the US military would deliberately reduce the effective range and lethality.
Australia is replacing the F88 Steyr based weapon with an unpdated one called the F90.
http://www.defense-aerospace.com/ar...f90-assault-rifle-in-low-rate-production.html
the M16 is a long weapon with a short batrrel, the AUG, the HK or even the L85 provide longer barrels in a shorter weapon. IIRC the carbine M16s are longer than the 'full fat' Aug or L85 and the carbine L85 (L22) has a similar length barrel to a standard M16