A Lower for the Lima Six Belt Fed Upper

Published on October 20, 2025
Duration: 7:51

This video details the design and testing of a prototype 3D-printed lower receiver intended for the Lima Six belt-fed upper. Key design considerations include a high dovetail for a short belt feed path to minimize jams and a two-piece construction for optimal print orientation and strength. The presenter discusses findings after 600 rounds, including issues with gas deflection and potential improvements for takedown pin placement and bolt catch functionality.

Quick Summary

A prototype 3D-printed lower receiver for the Lima Six belt-fed upper was tested after 600 rounds. Key design features include a high dovetail for a short feed path to prevent jams. However, issues like significant gas blowback and lack of a bolt catch were identified, prompting further design refinements.

Chapters

  1. 00:00Introduction to Prototype Lower
  2. 00:28Design Goal: High Dovetail
  3. 01:11Two-Piece Lower Construction
  4. 01:41Compromises: No Bolt Hold Open
  5. 01:58Selector Switch Incompatibility
  6. 02:16Lower Receiver Internals & Lubrication
  7. 03:00Upper Receiver Internals
  8. 03:28First Finding: Gas Blowback Issues
  9. 04:21Second Finding: Takedown Pin Orientation
  10. 05:10Overall Performance & Durability
  11. 06:17Manual of Arms: No Bolt Catch
  12. 07:07Future Feed Tray Loading Solutions

Frequently Asked Questions

What were the main design goals for the prototype lower receiver for the Lima Six belt-fed upper?

The primary design goal was to create a high dovetail mount for the belt sack to minimize the belt feeding path, thereby reducing the risk of jams. The lower was also designed with a two-piece construction to optimize print orientation for strength, particularly in the dovetail area.

What issues were encountered during testing of the prototype lower receiver?

The main issues included significant gas blowback into the shooter's face due to the support rib acting as a deflector, and the incompatibility with standard milspec selector switches, requiring push-button safeties. The lack of a bolt catch also made the manual of arms more complex.

How did the prototype lower receiver perform after 600 rounds of testing?

The prototype lower, printed from PLA Pro, held up well structurally and did not melt near the chamber, even after extended firing sessions. However, gas blowback issues and the absence of a bolt catch were noted as areas requiring further development.

What modifications are being considered for the prototype lower receiver?

Considerations include repositioning the support rib closer to the upper receiver, potentially adding venting or a seal to manage gas, flipping the takedown pin for easier access, and implementing a simple button to assist in closing the feed tray on a closed bolt.

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