i don't quite understand this...

bull barrels seem like a waste of money and materials... some of the most accurate AR's i've seen or shot have been SPR/Medcon profiles...

same goes for length... i've read a lot of chrono numbers for plenty of different brands/weights of ammo... and it seems like after 20"... you are adding length and not getting much more return on velocity...

are these fudd carry-overs into the AR world?

Bull barrels are highly accurate (if well built).  Not a waste of money....  the way a bull barrel vibrates (or rather doesn't vibrate), allows for slightly better accuracy.  Those who shoot nats-ass will usually get slightly better results from a bull.  I use a 24" varmint/bull barrel to elicit sub 1/2 moa groups (1/4" on a good day) 10 shot groups at 100 yards.  I've never seen a lightweight contour do that... may just be me.  As far as velocity... I agree... the differences aren't exactly huge after 18" .  However, my 16" is noticeably different compared to my 24" at REALLY long range.