I have a couple of 30 round mags for my HK-91. As mentioned earlier, they really are too long if you are shooting in the prone position. Plus being unusually long, there are not a lot of magazine pouches that they would fit into properly.
If somebody were to make a .308 HK-91 magazine similar to the Surefire Mag5-60
a more manageable 30, or 30+ round magazine could probably work... but they only make those for .223 as far as I know.
Edit: Ah, I see the surefires got mentioned on page two. I posted this before going to page two. D'oh!