All of my current uppers have barrels with 1/8 or 1/7 twist. They seem to like 69-77 grain bullets although that may be because the rounds I shoot with heavier bullets use higher quality bullets and are generally handloaded.
I use bulk 55 grain ammo (mostly Federal American Eagle bulk pack) in short range matches because I shoot a lot of it, it's cheap and at short range it's sufficiently accurate. But I don't generally get better than 1.5 MOA 5 shot groups from the bulk ammo, whereas I get sub MOA with 75 grain handloads. But maybe it's due more to quality than bullet weight.
I haven't had any reason to load some high quality light bullet loads and test them.
