I am in the minority here but it is my opinion that the FN PS90 is the best HD rifle. It is short and fires what some would consider a pistol round. I call it an intermediate intermediate round since it resembles a smaller 5.56. You are not going to have over penetration since it the only ammo civilians can get are the plastic tipped ones. The bullet is pretty light as well so it will lose speed fast through walls or flesh.50 round capacity is pretty good too.
The PS90 has nearly every advantage General Geoff pointed out about the AR. Of those he pointed out here are the ones the PS90 also possesses.
very reliable30-rd standard capacity
very light recoil
reduced overpenetration concerns
fairly short and maneuverable package (with shorter barrel lengths)
As you can see it retains all of the ARs best advantages and even has some better advantages such as shorter length and higher capacity.Now the AR is a good all around platform and HD is definitely one of its fortes. I think the PS90 is better in a pure HD role and the AR is the much more versatile option.