Daryl,
I don't think that most steel shot wads will work satisfactory because of their tapered shotcup construction.
If you have a fully rifled barrel gun try the Federal 12S3 or 12S4 (depending on slug length) that have parallel petals.
Another testing of the US-S 515P slug against 57mm thick bullet resistant glass. Holes are at about 12cm apart.

Lefteris
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