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A lightweight and low cost semi-automatic rifle includes an upper receiver, a lower receiver, and a forend fabricated from injection-molded polymers. The rifle permits firing .22 LR or similar low-power ammunition from an AR-15 style frame, operates on the blowback principle, and provides a fully fu

A lightweight and low cost semi-automatic rifle includes an upper receiver, a lower receiver, and a forend fabricated from injection-molded polymers. The rifle permits firing .22 LR or similar low-power ammunition from an AR-15 style frame, operates on the blowback principle, and provides a fully functional bolt catch and extractor/ejector/deflector.

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1. A rifle frame comprising: a receiver having an inner surface defining a chamber that extends from a rearward opening to a breech passage, the rearward opening and the breech passage being disposed coaxially along a central axis of the chamber and the breech passage being smaller in diameter than

1. A rifle frame comprising: a receiver having an inner surface defining a chamber that extends from a rearward opening to a breech passage, the rearward opening and the breech passage being disposed coaxially along a central axis of the chamber and the breech passage being smaller in diameter than the chamber, and including a lower opening extending radially from the chamber through a lower side of said receiver rearward from the breech passage;a barrel enclosing a bore extending from a breech end of the barrel to a muzzle end of the barrel, said barrel including an externally-threaded portion smaller in outer diameter than the breech passage of said receiver and disposed proximate the breech end of said barrel, a shoulder larger in diameter than the breech passage and disposed substantially adjacent to the externally-threaded portion, and a feed ramp extending downward and rearward from the breech end of said barrel adjacent to the bore; anda barrel nut having an internally threaded opening dimensioned to engage the externally-threaded portion of said barrel, the barrel nut being larger in outer diameter than the breech passage of said receiver,wherein the externally-threaded portion of said barrel is inserted through the breech passage of said receiver and said receiver is clamped against the shoulder of said barrel by threaded engagement of said barrel nut onto the externally-threaded portion of said barrel to sandwich the receiver between the shoulder and the barrel nut. 2. The rifle frame as claimed in claim 1, wherein the inner surface of said receiver includes laterally-opposed guide slots extending from the rearward opening to the breech passage substantially parallel to the central axis of the chamber. 3. The rifle frame as claimed in claim 2, wherein the shoulder of said barrel includes radially-protruding ribs fitted into the guide slots formed along the inner surface of said receiver, the ribs and the guide slots cooperating to align the feed ramp of said barrel with the lower opening of said receiver. 4. The rifle frame as claimed in claim 3, wherein said barrel further includes radial notches indented into the breech end of said barrel in alignment with the radially-protruding ribs formed on the shoulder of said barrel. 5. The rifle frame as claimed in claim 1, wherein the inner surface of said receiver also includes an upwardly-indented recess extending from the rearward opening toward the breech passage substantially parallel to the central axis of the chamber. 6. The rifle frame as claimed in claim 5, wherein the outer surface of said receiver includes an upwardly-protruding rail enclosing the upwardly-indented recess formed in the inner surface of said receiver. 7. The rifle frame as claimed in claim 1, further comprising a forend clamped between said receiver and said barrel nut, said forend extending forward from said receiver and surrounding at least a portion of said barrel. 8. The rifle frame as claimed in claim 7, the outer surface of said receiver including a main portion extending forward from the rearward opening to surround at least a rearward portion of the breech passage and including an annular boss protruding forward from the main portion to surround the remainder of the breech passage, said forend including a circular recess indented into the rearward end of said forend for assembly onto the annular boss of said receiver. 9. The rifle frame as claimed in claim 8, the outer surface of said receiver further including a rail protruding upward along the main portion of the outer surface and including tapered tabs protruding radially outward from the annular boss adjacent to the main portion of the outer surface, said forend further including tapered indents indented radially outward from the circular recess for assembly onto the tapered ribs, said forend including at least one rail protruding outward along said forend and the tapered indents and the tapered ribs cooperating to align the rail of said forend to the rail of said receiver. 10. The rifle frame as claimed in claim 1, said barrel including a reduced-outer-diameter portion immediately adjacent to the breech end of said barrel, the shoulder of said barrel being formed by a breech sleeve fastened onto the reduced-outer-diameter portion, and the externally-threaded portion of said barrel being disposed muzzleward from the reduced-outer-diameter portion, wherein said breech sleeve includes the feed ramp. 11. The rifle frame as claimed in claim 1, said receiver including an upper portion enclosing the chamber and the breech passage with a lug protruding from a lower surface of the upper portion adjacent to the breech passage, a lower portion enclosing an action group cavity with an ear protruding from a forward surface of the lower portion, and a pin inserted through the lug and the ear to pivotally connect the upper portion to the lower portion, the chamber and the action group cavity being mutually opened to each other and the lower opening being formed through the lower portion of said receiver between the action group cavity and the ear. 12. The rifle frame as claimed in claim 1, further comprising a breech insert molded into the breech passage of said receiver. 13. The rifle frame as claimed in claim 12, said receiver being formed of a first material that is relatively malleable and impact-resistant as compared to a second material from which said breech insert is formed. 14. The rifle frame as claimed in claim 1, wherein the externally-threaded portion of said barrel is disposed between the breech end of said barrel and the shoulder of said barrel. 15. A rifle comprising: a rifle frame having a receiver with an inner surface defining a chamber that extends from a rearward opening to a breech passage, the rearward opening and the breech passage being disposed coaxially along a central axis of the chamber and the breech passage being smaller in diameter than the chamber, and including a lower opening extending radially from the chamber through a lower side of said receiver rearward from the breech passage;a barrel enclosing a bore extending from a breech end of the barrel to a muzzle end of the barrel, said barrel including an externally-threaded portion smaller in outer diameter than the breech passage of said receiver and disposed proximate the breech end of said barrel, a shoulder larger in diameter than the breech passage and disposed substantially adjacent to the externally-threaded portion, and a feed ramp extending downward and rearward from the breech end of said barrel adjacent to the bore; anda barrel nut having an internally threaded opening dimensioned to engage the externally-threaded portion of said barrel, the barrel nut being larger in outer diameter than the breech passage of said receiver,wherein the externally-threaded portion of said barrel is inserted through the breech passage of said receiver and said receiver is clamped against the shoulder of said barrel by threaded engagement of said barrel nut onto the externally-threaded portion of said barrel,wherein the inner surface of the receiver of said rifle frame includes laterally-opposed guide slots extending from the rearward opening to the breech passage substantially parallel to the central axis of the chamber and includes sidewalls enclosing a lower cavity below the guide slots, the shoulder of the barrel of said rifle frame includes radially-protruding ribs fitted into the guide slots formed along the inner surface of the receiver, the ribs and the guide slots cooperating to align the feed ramp of the barrel with the lower opening of the receiver, and the barrel further includes radial notches indented into the breech end of the barrel in alignment with the radially-protruding ribs formed on the shoulder of the barrel;a bolt group including a buffer, guide rails extending along and forward from the buffer, a bolt slidingly mounted on the guide rails, a firing pin slidingly housed within the bolt, a recoil rod slidingly housed within the bolt and releasably clipped to the buffer, and a recoil spring captured on the recoil rod between the buffer and the bolt, said bolt group being inserted into said rifle frame with the guide rails fitted into the guide slots of the receiver of said rifle frame and into the radial notches of the barrel of said rifle frame, and the bolt being movable along the guide rails from a battery position distal from the buffer and proximate the shoulder of the barrel of said rifle frame to a charged position proximate the buffer;an action mounted in the lower cavity of the receiver of said rifle frame and including a hammer pivotally mounted on a first pin extending between the sidewalls of the receiver for motion between a cocked position and a discharged position, a hammer spring connected between the hammer and one of the sidewalls of the receiver for biasing the hammer toward the discharged position, a sear pivotally mounted on a second pin extending between the sidewalls of the receiver for motion between a locked position restraining the hammer and a released position not contacting the hammer, a sear spring connected between the sear and one of the sidewalls of the receiver to bias the sear toward the locked position, and a trigger pivotally mounted on a third pin extending between the sidewalls of the receiver, the trigger being movable to force the sear out of the locked position, said action being disposed with respect to said bolt group such that motion of the bolt of said bolt group from the battery position to the charged position moves the hammer and the sear to the cocked and locked positions;a bolt catch pivotally mounted in the receiver of said rifle frame and movable between a retracted position and an extended position in which said bolt catch restrains the bolt of said bolt group in the charged position; anda magazine releasably inserted into the lower opening of the receiver of said rifle frame such that motion of the bolt of said bolt group from the charged position to the battery position feeds a round from said magazine into the bore of the barrel of said rifle frame, said magazine including a case housing a follower and a spring for urging the follower toward an emptied position at the top of the case, the follower having a rearwardly-protruding hook, wherein the rearwardly-protruding hook moves said bolt catch to the extended position when the follower is in the emptied position. 16. The rifle as claimed in claim 15, wherein the follower of said magazine is dimensioned to carry one or more .22 LR rounds in a single stack and the case of said magazine includes a lower portion dimensioned to appear as a 5.56 mm NATO magazine and includes an upper portion dimensioned to closely fit the follower and to position an uppermost of the .22 LR rounds in line with said bolt group when said magazine is fully inserted into the lower opening of the receiver of said rifle frame, wherein the lower opening of the receiver of said rifle frame is dimensioned to releasably engage the lower portion of the case of said magazine, and wherein the feed ramp of the barrel of said rifle frame is angled and dimensioned to convey the uppermost of the .22 LR rounds from said magazine to the bore of the barrel when the bolt of said bolt group moves from the charged position to the battery position. 17. A rifle frame comprising: a receiver having an inner surface defining a chamber that extends from a rearward opening to a breech opening, the rearward opening and the breech opening being disposed coaxially along a central axis of the chamber, and including a lower opening extending radially from the chamber through a lower side of said receiver rearward from the breech passage;a breech insert molded into the breech opening of said receiver and having a breech passage formed therethrough, the breech passage being smaller in diameter than the chamber of said receiver, said breech insert including an internally threaded portion extending from the breech passage;a barrel enclosing a bore extending from a breech end of the barrel to a muzzle end of the barrel, said barrel including a breech end portion smaller in outer diameter than the breech passage of said breech insert and including a shoulder larger in diameter than the breech passage of said breech insert and disposed substantially adjacent to the breech end portion of said barrel; andan externally-threaded fastener engageable with the internally-threaded portion of said breech insert,wherein the breech end portion of said barrel is inserted through the breech passage of said breech insert and said breech insert is clamped against the shoulder of said barrel by threaded engagement of said externally-threaded fastener into the internally-threaded portion of said breech insert. 18. The rifle frame as claimed in claim 17, wherein the internally-threaded portion of said breech insert is disposed coaxially with and adjacent to the breech passage and said externally-threaded fastener is assembled over said barrel and directly contacts the shoulder of said barrel when threadedly engaged into the internally-threaded portion of said breech insert.