After seeing the praise for 1632 tubing on the Slingshot Forum I decided to try some myself. I mostly shoot smaller ammo (.177" BBs and 1/4" steel), so I figured it would work well. My favorite setup for .177" BBs is a pseudo-tapered setup, with about a 1 inch loop and 5-6 inches of single tube depending on your draw length. 6" singles also work splendidly.
For 1/4" steel ball bearings all you have to do is increase the loop size. I find a 1.5-2 inch loop with a 5-6" single is perfect for target shooting.
Along with this, Full 1632 loops have treated me well for plinking with 3/8" steel. Not enough power for anything other than target shooting but lots of fun and easy to make.
I also enjoy 2040 tubing, in either a single strand or small pseudo tapered setup for plinking with 3/8" steel. You could also shoot full looped 2040 but I find that to be too much power for 3/8 steel.