These are new Rob Allen Oil quenched carbon spring steel, They are designed to resist rust and do show a rust in time but its only like a stain, a lttle veg. oil wiped dry referbishes them well. 7.5 mm is just smaller than a 5/16" shaft, still a stiff, weighty shaft with good punch. They have a tri cut tip and I am not sure if they fly as striaght as a rock point. But at the close range of a sling it may not make much of a difference.
The Sling seconds have mold seam lines and minor defects on the surface. They can be sanded and camo painted if you want to make them look "pretty" but there is no difference in performance from the firsts. The light , medium, medium heavy, heavy, extra heavy, and super heavy bands all are available for it. Sure I would like to see those pics! Post them or send them e-mail.