A dry sprinkler (10, 10') comprising: an outer structural assembly having a proximal inlet, a distal outlet, and an internal passageway extending between the inlet and the outlet defining a longitudinal axis of the sprinkler (10, 10'), the outer structural assembly including: an inlet fitting (20, 20') including a proximal head portion and a distal body portion, the head portion having an external thread (24, 204) defining a nominal external thread diameter, the body portion including an external groove (26, 266) defining a nominal groove diameter being greater than the nominal external thread diameter, the external thread (24, 204) and groove (26, 266) providing the sprinkler (10, 10') with alternate threaded and grooved coupling arrangements for connection to a fluid supply pipe, the inlet fitting (20, 20') having an internal surface (22b,21) with four sections (I,II,III,IV), each of the four sections (I,II,III,IV) defining a different portion of the internal passageway, the first section defining an inlet portion with a sealing surface of the dry sprinkler (10,10'), the second section defining an expanding region of the internal passageway to a third section that defines the widest portion of the internal passageway, and a fourth section that converges the internal passageway in an axial-to-distal direction, the inlet fitting (20,20') having an outer surface portion that forms a transition between the proximal head portion and the distal body portion to define a transition portion between the external thread (24,204) and the external groove (26,266), the transition portion defining a stop surface; an outlet frame including a deflector axially spaced at a fixed distance from the outlet; and a casing tube disposed between the inlet fitting (20, 20') and an outlet frame; a seal assembly (108, 108') disposed along the passageway for contacting the sealing surface; and a thermal trigger for supporting the seal assembly (108, 108') against the sealing surface in an unactuated state of the dry sprinkler (10, 10'). an inlet fitting (20, 20') including a proximal head portion and a distal body portion, the head portion having an external thread (24, 204) defining a nominal external thread diameter, the body portion including an external groove (26, 266) defining a nominal groove diameter being greater than the nominal external thread diameter, the external thread (24, 204) and groove (26, 266) providing the sprinkler (10, 10') with alternate threaded and grooved coupling arrangements for connection to a fluid supply pipe, the inlet fitting (20, 20') having an internal surface (22b,21) with four sections (I,II,III,IV), each of the four sections (I,II,III,IV) defining a different portion of the internal passageway, the first section defining an inlet portion with a sealing surface of the dry sprinkler (10,10'), the second section defining an expanding region of the internal passageway to a third section that defines the widest portion of the internal passageway, and a fourth section that converges the internal passageway in an axial-to-distal direction, the inlet fitting (20,20') having an outer surface portion that forms a transition between the proximal head portion and the distal body portion to define a transition portion between the external thread (24,204) and the external groove (26,266), the transition portion defining a stop surface; an outlet frame including a deflector axially spaced at a fixed distance from the outlet; and a casing tube disposed between the inlet fitting (20, 20') and an outlet frame; a seal assembly (108, 108') disposed along the passageway for contacting the sealing surface; and a thermal trigger for supporting the seal assembly (108, 108') against the sealing surface in an unactuated state of the dry sprinkler (10, 10').