A firearm as disclosures herein comprises a main receiver body, a barrel, a breech bolt structure, a buffer assembly, a trigger group assembly, a bolt catch and at least one of an auto-sear, a magazine adapter and a breech bolt anti-rotation structure. The main receiver body is preferably of a monolithic construction whereby the handguard and breech bolt carrying bore can be manufactured from a single piece of material. The auto-sear is tripped by engagement with a buffer of the buffer assembly. The magazine adapter has an exterior surface structure engaged with a mating interior surface structure of a magazine well space of the main receiver body. A distal end portion of the breech bolt anti-rotation structure is engageable by the buffer for limiting forward travel of the buffer when the barrel is detached from the main receiver body.

A firearm as disclosures herein comprises a main receiver body, a barrel, a breech bolt structure, a buffer assembly, a trigger group assembly, a bolt catch and at least one of an auto-sear, a magazine adapter and a breech bolt anti-rotation structure. The main receiver body is preferably of a monolithic construction whereby the handguard and breech bolt carrying bore can be manufactured from a single piece of material. The auto-sear is tripped by engagement with a buffer of the buffer assembly. The magazine adapter has an exterior surface structure engaged with a mating interior surface structure of a magazine well space of the main receiver body. A distal end portion of the breech bolt anti-rotation structure is engageable by the buffer for limiting forward travel of the buffer when the barrel is detached from the main receiver body.

A firearm as disclosures herein comprises a main receiver body, a barrel, a breech bolt structure, a buffer assembly, a trigger group assembly, a bolt catch and at least one of an auto-sear, a magazine adapter and a breech bolt anti-rotation structure. The main receiver body is preferably of a monolithic construction whereby the handguard and breech bolt carrying bore can be manufactured from a single piece of material. The auto-sear is tripped by engagement with a buffer of the buffer assembly. The magazine adapter has an exterior surface structure engaged with a mating interior surface structure of a magazine well space of the main receiver body. A distal end portion of the breech bolt anti-rotation structure is engageable by the buffer for limiting forward travel of the buffer when the barrel is detached from the main receiver body.

A last round hold open (LRHO) apparatus comprising a first lever having a first end and a second end; a second lever having a first end and a second end, the first end of the second lever in contact with the second end of the first lever; a bolt catch in contact with the second end of the second lever, and with a bolt carrier; and a follower in contact with the first end of the first lever, such that when the follower pushes up on the first end of the first lever, the second end of the first lever pushes down on the first end of the second lever, and the second end of the second lever pushes up on the bolt catch, resulting in activation of the bolt catch and opening the bolt carrier to alert an operator of the firearm that the firearm is out of ammunition.

A last round hold open (LRHO) apparatus comprising a first lever having a first end and a second end; a second lever having a first end and a second end, the first end of the second lever in contact with the second end of the first lever; a bolt catch in contact with the second end of the second lever, and with a bolt carrier; and a follower in contact with the first end of the first lever, such that when the follower pushes up on the first end of the first lever, the second end of the first lever pushes down on the first end of the second lever, and the second end of the second lever pushes up on the bolt catch, resulting in activation of the bolt catch and opening the bolt carrier to alert an operator of the firearm that the firearm is out of ammunition.

A replaceable slide notch system for a firearm that includes a replaceable slide notch component dimensioned to fit within a corresponding cavity of a slide. The replaceable slide notch component includes a slide lock notch surface that locks the slide when the firearm's ammunition is exhausted. A second replaceable slide notch component has a second slide lock notch surface that locks the slide at a further rearward position than that of the first slide notch component, thus providing greater chambering/operating force when the slide is released.

A replaceable slide notch system for a firearm that includes a replaceable slide notch component dimensioned to fit within a corresponding cavity of a slide. The replaceable slide notch component includes a slide lock notch surface that locks the slide when the firearm's ammunition is exhausted. A second replaceable slide notch component has a second slide lock notch surface that locks the slide at a further rearward position than that of the first slide notch component, thus providing greater chambering/operating force when the slide is released.

A firearm as disclosures herein comprises a main receiver body, a barrel, a breech bolt structure, a buffer assembly, a trigger group assembly, a bolt catch and at least one of an auto-sear, a magazine adapter and a breech bolt anti-rotation structure. The main receiver body is preferably of a monolithic construction whereby the handguard and breech bolt carrying bore can be manufactured from a single piece of material. The auto-sear is tripped by engagement with a buffer of the buffer assembly. The magazine adapter has an exterior surface structure engaged with a mating interior surface structure of a magazine well space of the main receiver body. A distal end portion of the breech bolt anti-rotation structure is engageable by the buffer for limiting forward travel of the buffer when the barrel is detached from the main receiver body.

A last round hold open (LRHO) apparatus comprising a first lever having a first end and a second end; a second lever having a first end and a second end, the first end of the second lever in contact with the second end of the first lever; a bolt catch in contact with the second end of the second lever, and with a bolt carrier; and a follower in contact with the first end of the first lever, such that when the follower pushes up on the first end of the first lever, the second end of the first lever pushes down on the first end of the second lever, and the second end of the second lever pushes up on the bolt catch, resulting in activation of the bolt catch and opening the bolt carrier to alert an operator of the firearm that the firearm is out of ammunition.

A last round hold open (LRHO) apparatus comprising a first lever having a first end and a second end, a mag stop component to which the first lever is pivotally affixed at the second end, a second lever disposed proximate the second end of the first lever, the second lever pivotally affixed to the mag stop, a bolt catch affixed to the mag stop and the second lever, and a magazine release extending from the first end of the first lever.