We do not recommend the use of steel cased ammunition in the Mini. Steel cased ammunition in any rifle can cause bore damage and the lacquer finish can start coating the chamber causing extraction/ejection problems.
Unless their barrels are just as soft a steel as the cartrage caseing (they are not) it will tear up the caseing long before it will do damage to a bore. Unless you have a overloaded round that bulges the bore (not likely with steel cased ammo either as it is usualy a mild loading)
Did you tell them what kind of ammo you used? Sliver Bear is zinc coated not lacquer.