Scout rifles are bullpup and the auto rifles are not. Though the pulse rifles are also bullpup (in a much more realistic/traditional sense). Technically, the barrels on all of them should be about the same (though that's certainly not a rule, just a generalization), but scout and pulse rifles should be shorter than auto rifles overall. That was actually one of the only issues with the human arsenal in Halo. The Assault Rifle was obviously meant to be bullpup, but it was absolutely massive. It made the bullpup design superfluous. They obviously chose it because bullpup = futuristic.