That is nonsense. The 9mm specs have changed very little since WW1 If the gun ejected more than one spent case then there is very little chance "the chamber is too deep"!
Now another matter would be head space on a replaced barrel. The quickest way to look for this would be to examine a spent case after firing. A poorly supported case will show a slight bulge right at its base, just above the extractor groove in the case.
While inspection by a qualified gunsmith may be in order, I still suspect your sear is the culprit here.
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