Armorers Course

LIMA SIX Armorers Course

⚙️ LIMA SIX Armorers Course

Advanced — Gunsmiths & Armorers

1. Overview
2. Gas System
3. Buffer & Spring
4. Lower & Trigger
5. Break-In
6. Troubleshooting
7. Final Test

Module 1: System Overview

The LIMA SIX is a belt-fed upper receiver system designed to mount on a standard AR-15 lower receiver. Unlike traditional box-magazine AR uppers, the LIMA SIX feeds from a disintegrating-link belt, introducing unique mechanical considerations that armorers must understand thoroughly.

Key Differences from Standard AR Uppers

  • Belt feeding via a feed tray and feed pawl mechanism replaces the standard magazine well interface
  • The carrier and bolt are purpose-built for belt-fed operation
  • Timing and gas system tuning are more critical than in standard AR platforms
  • A mandatory break-in procedure is required before full-auto or FRT operation
  • Lubrication requirements are more aggressive than standard AR platforms — grease is preferred over oil

Build Kit vs. Complete Upper

The LIMA SIX is sold as either a build kit (requiring armorer assembly and gas port verification) or a complete upper (pre-assembled and validated). Build kit armorers must verify gas port sizing, gas block alignment, and buffer compatibility before declaring the system ready. Complete uppers skip the gas system checks and proceed directly to buffer configuration.

Armorer Note Gas port sizing is the most common source of reliability issues in build kits. Always measure and confirm before test-firing. Complete uppers ship with gas ports intentionally sized — do not alter without guidance from LIMA SIX.

Supported Lower Receivers

  • Mil-Spec AR-15 or M16
  • Hydra Weaponry HYDRA 6
  • Inoculum Group IGM7
Note — Older Units Some older Hydra Weaponry HYDRA 6 lower receivers may experience a compatibility issue with the Atrius Forced Reset Selector. Hydra has addressed this in more recent production. If a customer reports issues with this combination, direct them to contact Hydra Weaponry.

Module 2: Gas System

Gas Port Sizing

Gas port size is the most critical variable in build kit assembly. The correct diameter depends on gas system length and barrel length. Undersized ports cause under-gassing and cycling failures. Oversized ports cause over-gassing and accelerated wear — if a port is oversized, an adjustable gas block must be used to regulate flow.

LIMA SIX Gas Port Size Chart

Gas Block Types

Fixed Gas Block

The preferred choice for rifle-length gas systems and the simplest overall. Gas port sizing must be correct since there is no adjustment capability. Ensure proper alignment to the gas port.

Rifle-Length Gas Systems and Adjustable Gas Blocks A fixed gas block is strongly recommended for rifle-length builds. Adjustable gas blocks may be slightly restrictive even at maximum setting and are more prone to leakage — both problems are amplified by the already-lower gas pressure in rifle-length systems.

Riflespeed Adjustable Gas Block

  • For initial setup and break-in, start with the shortest plunger: .887″. The factory-installed plunger is .917″ and is generally too long for this configuration.
  • Once broken in, sizing up to .897″ or .907″ may be necessary to prevent overgassing, particularly when running suppressed. Note: LIMA SIX factory complete uppers intentionally use .897″ because the gas port is oversized by .005″.
  • Set to position 12 (maximum gas) to start. Reduce only once the gun is cycling consistently.
  • Verify the plunger moves freely. If stuck: disassemble, clean, and lubricate.
  • Riflespeed blocks inherently leak slightly due to moving parts — may require increasing gas port by ~.005″.

Superlative Arms Adjustable Gas Block

  • Set to maximum gas to start: fully open = 18 clicks CCW from fully closed.
  • Reduce gas setting only once the gun is cycling consistently.

Other Adjustable Gas Blocks

Set to maximum gas setting to start. Reduce only once the gun is cycling consistently. Ensure proper alignment to the gas port.

Rifle-Length Gas System — Additional Requirements

Hardware Requirements

  • H2 buffer weight — H3 is not supported on rifle-length builds
  • Black spring (KAK 308 Carbine) — Sprinco Orange is not compatible
  • A5 or Rifle-length tube only — a Carbine tube creates too much spring preload even with the Black spring

Operational Notes

  • Requires perfect gas block alignment
  • High-quality ammunition required — M193 spec preferred
  • May require additional break-in beyond the standard 200 rounds
  • A high back-pressure suppressor may improve reliability

Gas Tube Installation

Two common installation errors cause complete gas failure:

  • Upside-down installation: The gas hole in the tube must be oriented toward the barrel, not away from it.
  • Wrong tube length: The gas tube must protrude approximately one inch into the receiver. An incorrect length results in zero gas reaching the carrier.

Module 3: Buffer, Spring & Spacer System

The LIMA SIX buffer system is more complex than a standard AR platform. The recoil spring type, buffer tube length, buffer selection, buffer weight, and optional spacers all interact. The spring preload is affected by tube length — a Black spring in a Carbine tube has significantly higher preload than the same spring in a Rifle-length tube. For this reason, the correct combination must be selected as a system, not as individual components.

Key Components

  • L6 Spring Booster: Increases the preload of the Black spring to approximate the tension of the Sprinco Orange spring.
  • L6 Rifle Tube Spacer (~2″): Shortens a Rifle-length tube to approximate the effective length of an A5 tube, reducing spring preload.
  • Kurt Spacer: A single-piece combination of the Rifle Tube Spacer and Spring Booster.

Supported Buffer Configurations

LIMA SIX Buffer Configuration Chart
2.5″ AR10 Buffer Restriction The standard AR10 Buffer (2.5″) in a Carbine tube supports semi-auto only. It is incompatible with Full Auto and FRT / Super Safety triggers. The Tromix L6 (also 2.5″) does not carry this restriction and supports all trigger types.
AR15 Rifle Buffer (5.9″) This buffer is not compatible with any LIMA SIX configuration and should never be installed.

Rifle-Length Gas System Buffer Requirements

For rifle-length gas systems, a weaker spring preload is required due to the lower gas pressure. The two recommended configurations are:

  • AR15 Buffer (3.25″) / H2 / Black Spring / No Spacer / A5 (7.75″) tube
  • AR15 Buffer (3.25″) / H2 / Black Spring / L6 Rifle Tube Spacer / Rifle (9.68″) tube

Note: The Black spring must be installed in an A5 or Rifle-length tube for rifle-length gas systems — a Carbine tube creates too much preload even with the Black spring.

FRT / Super Safety Spring Requirements

FRT and Super Safety triggers generally benefit from a stiffer spring preload. This is achieved through either the Sprinco Orange spring or the Black spring with a Spring Booster. The correct combination depends on the tube length — consult the supported configurations table above. When running an FRT on a rifle-length gas system, use the weak spring configuration (rows 10–11 above) and ensure the system is fully broken in, well lubricated, using quality ammunition, and avoid long unsupported belts.

Module 4: Lower Receiver & Trigger Systems

Semi Auto

Standard mil-spec semi-auto triggers are supported without restriction. Buffer weight verification and timing video review are not required for semi-auto configurations.

Full Auto

Requires completion of the 200-round break-in in semi-auto mode before full-auto operation.

FRT / Super Safety Triggers

  • Light trigger pull is mandatory. Excess force creates drag on the bolt carrier — a primary cause of FRT malfunctions.
  • 200-round break-in in semi-auto must be completed before FRT operation
  • Spring selection must match tube length (see Module 3)

FRT / Super Safety Device Types

DeviceSpecial Notes
Hoffman Style Super Safety—
Hoffman Trigger Kicker—
AS Designs ARC FireMust use trigger provided with the unit — no substitutes or DIY modifications
Atrius Forced Reset SelectorMay be incompatible with some older Hydra HYDRA 6 lowers — direct to Hydra if issues arise
Arizona Regulator—
Triggered Disruptor (Formerly Partisan)Increased carrier drag even in semi-auto mode — avoid during break-in
Rare BreedIncreased carrier drag even in semi-auto mode — avoid during break-in
Delta Team TacticalIncreased carrier drag even in semi-auto mode — avoid during break-in
Cassette-Style FRTs During Break-In Cassette-style FRTs generate more hammer drag on the carrier compared to super safety-style devices, and this drag is present even in semi-auto mode. Avoid these triggers entirely during the 200-round break-in period — use a standard mil-spec trigger instead.

Module 5: Break-In Procedure

The 200-round break-in is mandatory before any Full Auto or FRT / Super Safety operation. It is the leading cause of reliability issues when skipped or shortcut.

Break-In is Non-Negotiable for Full-Auto / FRT Operation Document break-in completion in the unit's service record. A unit that has not completed break-in cannot be considered operationally ready for full-auto or FRT use.

Break-In Procedure

  1. Install a lightweight buffer. Do NOT use a heavy buffer (H1–H3) during break-in.
  2. Semi-auto only with a mil-spec trigger. Cassette-style FRT triggers must not be used during break-in even in semi-auto mode — they create additional carrier drag that interferes with the process.
  3. Lubricate liberally with grease as specified in the operators manual.
  4. If using an adjustable gas block, set to maximum gas setting.
  5. Use short belts or hand-support the belt to prevent belt weight from interfering with cycling.
  6. Fire 200 rounds total in semi-auto mode.
O-Ring Note Removing the o-ring from the extractor spring during break-in may improve bolt closure reliability. If removed, it must be reinstalled after break-in is complete.

Post Break-In Setup

  • Install the correct operational buffer and spring combination for the configuration (see Module 3)
  • Watch the LIMA SIX Timing Video before declaring the unit ready for full-auto / FRT operation
  • Reinstall o-ring if removed during break-in

Last-Round Flyer

When evaluating accuracy, exclude the last round from each belt. The delinking process can slightly deform the last round, causing a displaced impact. This is a normal characteristic of all belt-fed systems and is not a LIMA SIX defect.

Accuracy Troubleshooting

  • Try a different brand or type of ammunition
  • Inspect and clean the rifling — carbon buildup can degrade accuracy
  • Always exclude the last round when evaluating groups

Module 6: Armorer Troubleshooting Guide

Failure to Fully Chamber — Loading First Round

  • Check lubrication (refer to operators manual lubrication video)
  • Remove o-ring from extractor spring during break-in to test for improved bolt closure — reinstall afterwards
  • Verify both ejectors can be depressed flush to the bolt face
  • Inspect extractor for burrs; polish if present
  • Deburr the front left edge of the feed tray with fine-grit sandpaper or emery cloth

Failure to Fully Chamber During Firing / Undergassed

  • Verify adjustable gas block is at maximum setting
  • If Riflespeed: confirm plunger moves freely; if stuck, disassemble, clean, lubricate. Confirm .887″ plunger installed and set to position 12
  • Test with different ammunition (5.56×45 M193 spec recommended)
  • Check lubrication
  • Remove o-ring from extractor spring during break-in — reinstall afterwards
  • Verify ejectors can be depressed flush to bolt face
  • Inspect extractor for burrs; polish if present
  • Check gas key for looseness — wiggle firmly; it must not move at all
  • Deburr front left edge of feed tray

Failure to Fire — Semi Auto

  • Inspect hammer spring for upside-down installation
  • Test with different ammunition

Failure to Fire — Full Auto / FRT / Super Safety

  • Suspected bolt bounce or timing issue
  • Direct operator to watch LIMA SIX Timing Video

Zero Gas (Carrier Does Not Move)

  • Inspect gas key on bolt carrier — remove any stuck primer
  • Verify gas tube is not installed upside-down (gas hole must face the barrel)
  • Verify gas tube length — must protrude approximately 1 inch into receiver

Accuracy Issues

  • Exclude last round from group evaluation (last-round flyer phenomenon)
  • Try different ammunition
  • Inspect and clean rifling in barrel
Contact LIMA SIX Phone: 215-956-0700  |  Email: contact.us@limasix.com
Hours: Monday–Thursday, 8am–6pm Eastern

Module 7: Armorer Certification Test

25 questions — multiple choice. A score of 80% or higher (20/25) is required to pass.

1What is the recommended gas port size for a Carbine-length gas system with a 14.5″ barrel?
2A Rifle-length gas system build requires which recoil spring, and in which tube?
3What does the L6 Spring Booster do?
4The 2.5″ AR10 Buffer in a Carbine-length tube is restricted to which trigger type?
5For initial setup of a Riflespeed adjustable gas block on a build kit, which plunger length should be installed?
6When should an adjustable gas block be turned down from maximum gas?
7A Riflespeed gas block that may require enlarging the gas port by approximately .005″ is exhibiting which known characteristic?
8Which lower receiver is incompatible with the Atrius Forced Reset Selector?
9How many rounds are required for the mandatory LIMA SIX break-in, and in which fire mode?
10Which buffer weight is correct for a Rifle-length gas system?
11Why must cassette-style FRT triggers (e.g. Rare Breed, Triggered Disruptor) be avoided during the break-in period?
12What gas port size is recommended for a Mid-length gas system with a 16″ or 18″ barrel?
13An operator reports the carrier does not move at all when the rifle is fired. Which of the following is the correct initial diagnostic approach?
14The AS Designs ARC Fire has what unique trigger group requirement?
15What is the correct armorer response when a gas port is found to be oversized?
16How far must the gas tube protrude into the receiver for correct function?
17Which buffer should be installed during the 200-round break-in?
18An operator reports inconsistent accuracy with occasional flyers. Upon questioning, they are including the last round of each belt in their groups. What is the correct response?
19When troubleshooting failure to fire in Full Auto or FRT mode, what is the primary recommended action?
20Which gas system / barrel combination is NOT supported on the LIMA SIX?
21An armorer observes an abnormal feed pawl condition during inspection. What is the correct course of action?
22The Tromix L6 Buffer (2.5″) differs from the standard AR10 Buffer (2.5″) in which important way?
23What is the correct method for checking gas key integrity during a zero-gas diagnosis?
24A unit with a Riflespeed gas block is experiencing failure to chamber during firing. After confirming the gas block is at maximum setting, what is the next diagnostic step?
25Why do FRT / Super Safety triggers require a light trigger pull, and what happens if too much force is used?

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