Smoothbore question

[Tn poor boy]

The only way that I would attempt shooting steel shot would be to 1) Insure that there is “no choke”, a true cylinder boring. 2) Utilize a 20ga plastic shot cup with the appropriate sized cup to accommodate the volume of shot in your load. Use a fiber over-shot disc over the plastic cup. The key is to keep the steel shot off the barrel. If there is any choke(constriction), unlike lead, the steel resists confirming to the choke, and can damage the barrel.
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So you would melt plastic shoot cup in your smothbore. I would not do this not smart thing to do

 

[Brokennock]

So you would melt plastic shoot cup in your smothbore. I would not do this not smart thing to do

While I don't use them as my personal goal is to be as hc/pc as possible, plastic shot cups can be used in muzzleloading shotguns and fowling pieces without melting them.
All it takes is appropriate overpowder wads.