Who makes a light weight AR in 223 that is lightweight and has a 20" barrel?

Anyone offering an A2 or A4 mil contour 20 " bbl rifle.

If by lightweight you mean A1 "pencil" profile, Fulton Armory has a few listed. One is A1 retro, another has 1:8 and a rail. There's also the secondary market - like a SP-1.

Also there are a number of sellers who offer used or new uppers in that configuration.

i use mcgowen for my personal ARs

but i wouldn't do a 20" ultralight .. too long a lever for bending