Bullpup
A layout with the action and magazine behind the trigger, so a long barrel fits in a short weapon.
Moving the whole action behind the trigger group buys roughly 200 mm of overall length for the same barrel. A Steyr AUG is 790 mm long with a 508 mm barrel; a conventional rifle with that barrel is nearer a metre.
The costs are consistent across every design that has tried it. The trigger connects to the sear through a long linkage and feels worse than a conventional one. The ejection port is beside the firer’s face, so left-handed use needs a reversible bolt or a different ejection path. The balance is rearward, which some shooters prefer and some do not. And a magazine change happens behind the shooting hand, out of sight.
The FAMAS, the AUG, the QBZ-95 and the P90 on this site are all bullpups, and each answers the ejection problem differently.