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hollow-forge

American  
[hol-oh-fawrj, -fohrj] / ˈhɒl oʊˈfɔrdʒ, -ˈfoʊrdʒ /

verb (used with object)

hollow-forged, hollow-forging
  1. to produce (a tube or vessel) by trepanning a hole in a forging and expanding it with further forging on a mandrel.