Bolt assemblies for firearms

New and improved bolt assemblies for M4/M16 firearms capable of firing a remarkably greater number of rounds than previously available M4/M16 firearms are, disclosed that have right and left compression springs that are parallel to each other, straddle the firing pin bore and are retained in partial bores that have proximal and distal ramps descending from their open ends. Additionally, these bolt assemblies advantageously (A) have the extractor pivot pin receiving lug is positioned at the longitudinal center of the mass of the extractor, (B) walls of bolt lugs on the firearm bolt are angled tangent to full radius fillets joining adjacent bolt lugs, (C) the extractor contains a ceramic ball and a captive rubber spring to bias such ball against the extractor pivot pin to retain it in the receiving lug and (D) lugs on the barrel extension are angled tangent to substantial radius fillets joining them.