Here is the scoop on the #18 and the #22 rings. I know because I just went through this. The low #22 rings willl fit with the Vari-X III Leupold scopes. I corresponded with a guy that sent me pictures of his set up, #18 with the #22 low rings. The area that is tight is the ocular ring but it fits.
I had the medium #22 rings on my M1A and it was too high for my taste. I switched to QRW low rings and I am going to get some #22 lows in the near future. I want to be able to pull the scope and shoot the irons without changing anything. Right now I have an Eagle buttstock/cheek pad on my rifle and the #18 with the low QRW rings works out pretty good.
QRW rings are available in 30mm as well, as someone else pointed out. Mine are 1" for the Vari-X III 4.5-14 X 40mm that I have.