Just out of curiosity, how far are you going to be shooting, what and why an ultralight.
If ultralight I assume its a hunting rifle.
An ultralight 300 WB or 300 Rum is going to be a punishing to shoot hunting or otherwise.

You can shoot out to 1200-1300 yards with a 308 and 208 gr amax supersonic or with a 30-06, probably 1500 yards or so.

A 7mm round might be a better option like 284 win, 7mm WSM.
Even the 7mm-08 with the 162 gr amax can reach out to has about the same distance as the 208 gr 308 bullet, but with less recoil.

A while back I DRT'd a 250# hog at 300 yards with a 7mm-08 with the 162 gr amax.
Dropped him like a rock.

Not what I would consider long distance but it was a boar and they are tough animals so I was happy.