Would love to get some feedback from you of actual kills using marbles. They are my preferred ammo. Have you taken any game with marbles?
A lot of people say not to use marbles for hunting. But I have only recently started using steel because I can get them at work from old bearings.
I have hunted with 5/8 marbles for 12 years with great success. They are the same weight as 7/16 steel but not as dense, so not likely to penetrate. But the foot lbs are there. Just need a well placed shot. And as long as there going 240 FPS or above I've had no problem taking pigeons, grouse, squirrels, rabbits. Actually the squirrel I took yesterday with the modified cobra. Was light bands and a half inch marble. Neck shot, instant kill.
So steel/lead is better. But marbles will definitely do the job.