The guy on AR15 that is designing most of the crazy lowers has a special set of skills. He’s been working to build a Red Ryder tribute AR and posted these recently. The lower was something they released, and he went from there. Somebody asked him a question about the lever engaging the BCG and he said: The lever action reciprocating arm assembly does indeed interlock with the carrier. My original design allowed for the upper and lower receivers to pivot at the front pin just like any normal AR. Engaging the modified carrier of what would have been the buffer "now the feed arm assembly", together. However, due to the normal machining tolerances of AR parts, the slightest amount of play remained. Which I did not like at all. The push/pull of the arm and carrier "play", was not in the connection of the two parts, it was in the vertical play between the buffer tube/lower, and the upper receiver specs designed for carrier travel. Not unlike some piston carrier tilt issues, if that makes any sense. That design had a wobble at the tube and lower tube threads. My solution for this was to make sure the arm and carrier locked together in a way that would not allow for any vertical travel. If you pull both pins of an AR, keeping the upper and lower receivers together, the upper will slide forward just slightly inside the lower before the upper take-down legs contact lower internals, forcing a stop. This gap between the rear of the upper receiver and the lower buffer tube extension, is normally a space of .175-.195. While I cannot extend the carrier, I can extend the locking mechanics to protrude farther out of the buffer tube assembly (but no longer capable of front take down pin rotation). This allows the upper and lowers just enough space to fit together (upper being slightly forward of the lower), slide back into position, allowing the take-down pins to engage, seating the arm back inside the tube slightly, locking the two halves together just inside the buffer tube assembly. No more play, and also helping to ensure the action functions more smoothly. Thought some here might appreciate the project.