According to my extensive research (first hit on google) https://www.amazon.com/Chrome-Steel-Ball-Bearings-G25-25/dp/B01AX6VTFU
6mm steel ball weighs .88g
To use that weight, the gun needs to be quite powerful, over the firearm limits, to be able to put a reasonable backspin on it, IMO.
The other problem is that steel doesn't deform, it transfers all of the energy upon impact. Think of Newton's cradle (pic). Plastic BBs deform, even shatter upon impact that uses up a lot of its energy.
There were clear plastic Bioval BBs available until banned, long time ago. They were very rigid and they could shatter windshields and windows, even with their low weight, low speed.
I don't know, but I think a gold ball is harder than a BB and of course it doesn't shatter, but plastic deforms quite a lot.