Automatic firearm

An automatic firearm comprising:

• a receiver assembly having an upper receiver portion and a lower receiver portion demountably secured to said upper receiver portion, said upper receiver portion defining a longitudinally extending cavity;
• a barrel assembly demountably secured to the forward end of said upper receiver portion;
• a bolt assembly for disposed within said cavity reciprocation along the longitudinal direction thereof;
• a single compression spring disposed behind said bolt assembly in said cavity for maintaining reciprocation of said bolt assembly;
• a trigger group comprising:
• a pivotally mounted sear for engaging said bolt assembly;
• a pivotally mounted trigger;
• and
• a single linkage arm linking said trigger to said sear, said trigger and said sear each being pivotally connected directly to said linkage arm, whereby release of said trigger releases said sear from said bolt assembly acting through only said single linkage arm;
• wherein said lower receiver portion is formed as an integral unit enclosing said trigger group so as to expose only said trigger;
• means for vertically mounting a magazine to said upper receiver portion for feeding ammunition into said cavity in position to be chambered;
• and
• an ejector secured in fixed disposition to an interior side wall of said cavity for ejecting fired cases from said firearm, said upper receiver portion defining an ejection port opposite said ejector.