For bittering only Nugget does a great job, I've used it a lot. Magnum does an equally great job for bittering only. My favorite bittering only hop is Horizon, super smooth because of low cohumulone levels, so you can lay in more bitterness without it getting harsh. Nugget has the advantage because you can also use it for later additions contributing some nice floral fruitiness, in my experience.
I think there's probably alot more to bittering quality than cohumulone levels
What is perhaps not realized or appreciated enough by a lot of brewers is that there is a lot more to perceived bitterness than just alpha (or beta) acids, period. If you have ever done a whirlpool steep in the ~120-140F range in an otherwise unhopped wort, you will taste quite a bit of bitterness in the final beer. This indicates that there is a lot of "stuff" in hops independent of the acids that add to perceived bitterness. I don't know what that stuff is, but I know it's there. I suspect it's this other "stuff" that is responsible for quality differences in perceived bitterness, rather than cohumulone levels.
This has more to do with inadequacy of IBU calculators, not with some mysterious hop oils. A chemical reaction isn't prevented or doesn't stop just because the temperature is reduced a little. Isomerization continues at a slower rate. I can send you my IBU equation that includes whirlpool hop additions, stand by...
EDIT: On second thought... the isomerization rate at your very low whirlpool temperature of ~130 F becomes tiny and negligible, so maybe there really is something else entirely going on at that low temperature. But I think we can still both be correct...
FWIW, my formula is based on empirical utilization data (from Palmer), emulation of Tinseth, and my assertion that you'll get about half the IBUs at approximately
170 F compared to what you would get during the boil.
So, here it is.
IBU = oz * AA% * [sqrt(5*Boiltime)/V + sqrt(2*HStime)/V]
Calculate for each different hop addition, then take the sum. V is volume in gallons. Boiltime and HStime (Hop Stand time) need to be entered in minutes of course.
Regarding gravity effects:
This works best for worts of "normal" specific gravity of about 1.050-1.060, and warm temperatures >160 F -ish.
At higher gravity (e.g., >1.080), change the sqrt 5 to a 4 instead, and sqrt 2 to 1.5.
At low gravity (e.g., 1.035-1.040), change the 5 to a 6, and the 2 to 2.5.
Try it out, maybe you'll like it. My intent here was not so much a new IBU formula, so much as it is an alternative memorizable reference of how one might calculate IBUs on the fly using a few taps on a calculator. Cheers.