Firearm

1. In combination, a firearm having an upper receiver, an operating rod operably associated with said upper receiver for movement between a first position and a second position, a lower receiver connected to said upper receiver, a trigger mechanism carried by said lower receiver and a magazine connected to said lower receiver for carrying a plurality of ammunition cartridges, the improvement comprising:

(a) a barrel connected to said upper receiver, said barrel having a rear portion having a guide slot and a curved bearing surface and an intermediate portion having a gas transfer passageway formed therein;

(b) a generally semi-circular receiver bearing formed in said upper receiver;

(c) a bolt slidably received within said upper receiver for movement between a first forward position and a second rearward position, said bolt having a longitudinally extending guide slot;

(d) a firing pin carried by said bolt for movement between a first position and a second position;

(e) a striker connected to said bolt for movement between a first position and a second position in engagement with said firing pin;

(f) a locking member removably connected to said upper receiver for movement relative thereto between a first bolt engaging position and a second bolt disengage position, said locking member having a rearward portion, a central portion having a locking shoulder and a curved forward portion received within said generally semi-circular bore of said upper receiver and within said curved bearing surface of said barrel;

(g) locking member biasing means carried by said upper receiver for urging said locking member into said first bolt engaging position; and

(h) a disengage member slidably received within said longitudinally extending guide slot of said bolt and within said guide slot of said barrel for movement by said operating rod between a first position and a second position, said disengage member having a wall for engagement with said locking member to move said locking member into said bolt disengage position upon movement of said disengage member to said second position.

2. The firearm as defined in claim 1 in which said improvement further comprises bolt biasing means for yieldably resisting movement of said bolt between a first forward position and a second rearward position.

3. The firearm as defined in claim 1 in which said improvement further comprises striker biasing means connected to said striker for urging movement of said striker from said first position toward said second position in engagement with said firing pin.

4. The firearm as defined in claim 1 further including a piston housing connected to said barrel, a piston carried by said piston housing for engagement with said operating rod and a gas transfer passageway communicating with the barrel for transferring propellant gas pressure therefrom into said piston housing.

5. The firearm as defined in claim 1 in which said trigger mechanism comprises:

(a) a trigger housing;

(b) a lift bar pivotally connected to said trigger housing, said lift bar having a first end, a second end and an intermediate portion;

(c) a trigger pivotally connected to said trigger housing and operably associated with said lift bar for lifting said first end of said lift bar upon pivotal movement of said trigger; and

(d) a lifter member pivotally connected to said lift bar for movement between a first position and a second position for engagement with said sear to move said sear toward said second position of said sear.

6. In combination, a firearm having an upper receiver, a barrel connected to said upper receiver, an operating rod carried by and operably associated with said barrel for movement between a first position and a second position, a lower receiver connected to said upper receiver and a magazine connected to said lower receiver for carrying a plurality of ammunition cartridges, the improvement comprising:

(a) a generally semi-circular receiver bearing formed in said upper receiver;

(b) a bolt slidably received within said upper receiver for movement between a first forward position and a second rearward position, said bolt having a longitudinally extending guide slot;

(c) a firing pin carried by said bolt for movement between a first position and a second position;

(d) a striker connected to said bolt for movement between a first position and a second position in engagement with said firing pin;

(e) a locking member removably connected to said upper receiver for movement relative thereto between a first bolt engaging position and a second bolt disengage position, said locking member having a rearward portion, a central portion having a locking shoulder and a curved forward portion received within said generally semi-circular bore of said upper receiver;

(f) locking member biasing means carried by said upper receiver for urging said locking member into said first bolt engaging position;

(g) a disengage member slidably received within said longitudinally extending guide slot of said bolt for movement by said operating rod between a first position and a second position, said disengage member having a wall for engagement with said locking member to move said locking member into said bolt disengage position upon movement of said disengage member to said second position; and

(h) a sear operably associated with said striker, said sear being pivotally connected to said upper receiver for movement between first and second positions; and

(i) a trigger mechanism connected to said lower receiver, said trigger mechanism comprising:

i. a trigger housing;

ii. a lift bar pivotally connected to said trigger housing, said lift bar having a first end, a second end and an intermediate portion;

iii. a trigger pivotally connected to said trigger housing and operably associated with said lift bar for lifting said first end of said lift bar upon pivotal movement of said trigger; and

iv. a lifter member pivotally connected to said lift bar for movement between a first position and a second position to engage said sear to move said sear towards said second position of said sear.

7. The combination as defined in claim 6 in which said sear includes a shoulder for engaging said striker when said striker is in said first position.

8. The combination as defined in claim 6 in which said trigger includes a transversely extending pivot pin and in which said intermediate portion of said locking member is provided with a groove for receiving said transversely extending pivot pin.

9. A firearm comprising:

(a) an upper receiver having a barrel receiving bore, a pair of transversely spaced guide grooves and a generally semi-circular bore;

(b) a barrel having a forward portion and a rear portion having a curved bearing surface, said rear portion being telescopically receivable within said barrel receiving bore;

(c) a bolt slidably received within said transversely spaced guide grooves of said upper receiver for movement between a first forward position and a second rearward position, said bolt having a longitudinally extending guide slot;

(d) a striker connected to said bolt for movement between a first position and a second position in engagement with said firing pin;

(e) a firing mechanism associated with and carried by said bolt, said firing mechanism including a firing pin carried by said bolt for movement between a first rearward position and a second forward position;

(f) a locking member removably connected to said upper receiver for movement relative thereto between a first bolt engaging position and a second bolt disengage position, said locking member having a rearward portion, a central portion having a locking shoulder and a curved forward portion received within said generally semi-circular bore of said upper receiver and disposed in engagement with said curved bearing surface of said rear portion of said barrel;

(g) biasing means carried by said upper receiver for urging said locking member into said first bolt engaging position;

(h) a disengage member slidably received within said longitudinally extending guide slot of said bolt for movement between a first position and a second position, said disengage member having an angled wall for engagement with said locking member to move said locking member into said bolt disengage position;

(i) an operating rod carried by and operably associated with said barrel for movement between a first position and a second position in engagement with said disengage member for moving said disengage member into said bolt disengage position;

(j) a sear operably associated with said striker and pivotally connected to said upper receiver for movement between first and second positions;

(k) a lower receiver connected to said upper receiver;

(l) a trigger mechanism carried by said lower receiver and operably associated with said striker; and

(m) a magazine connected to said lower receiver for carrying a plurality of ammunition cartridges.