Overview
This lecture provides an overview of the M240B Medium Machine Gun, including its history, characteristics, assembly/disassembly, firing procedures, immediate and remedial actions, maintenance, and aiming devices.
Introduction & History
- The M240B is a key support weapon in the Marine Corps, replacing the M60E3 for greater range and reliability.
- Variants are used worldwide; the M240B is the primary ground-mounted version.
Characteristics
- Belt-fed, air-cooled, gas-operated, fully automatic, fires from the open bolt.
- Weighs 27.4 lbs; total system with tripod: 47.4 lbs; length: 48.5 inches.
- Rates of fire: sustained 100 rpm, rapid 200 rpm, cyclic 650 rpm.
- Max range: 3,725 m; effective suppression: 1,800 m; grazing fire: 600 m.
- Eight main components: barrel, butt-stock/buffer, drive spring rod, bolt/operating rod, trigger housing, cover, feed tray, receiver.
Operating Procedures
- Safety: Left to right (S visible) = safe; right to left (F visible) = fire; weapon must be cocked to engage safety.
- Cycle of operation: feeding, chambering, locking, firing, unlocking, extracting, ejecting, cocking.
Ammunition
- Issued in 100-round bandoleers; basic allowance: 400 rounds per gun.
- Types: Dummy (training), Blank (simulated), Ball (standard), Tracer (marking), Armor-piercing.
Loading, Unloading, and Clearing
- Check if barrel is hot (>200 rounds in 2 minutes); keep cover closed and wait to cool if so.
- Steps: Lock bolt to rear, place on safe, remove ammo, inspect chamber, close cover, ease bolt forward.
Disassembly & Assembly
- Field strip into five groups: barrel, buttstock/buffer, drive spring rod, bolt/operating rod, trigger housing.
- Assembly involves reversing the disassembly steps and performing a function check.
Care and Cleaning
- Use CLP, RBC, and other approved lubricants.
- Clean after firing for three days to remove carbon.
- Avoid cleaning agents on nonmetallic parts; inspect daily in combat or field conditions.
Mounts and Accessories
- Sights: Adjustable front/rear for 200–1,800 m, bipod (non-removable), M122 tripod for stability.
- T&E mechanism enables precise targeting adjustments.
- Gun and accessories carried in a gun bag with spare barrel bag.
Barrel Changing Procedures
- Change after 10 min sustained or 2 min rapid fire; minimum two, max seven clicks when seating new barrel.
Malfunctions & Stoppages
- Malfunctions: sluggish operation (dirt/lube issues), runaway gun (worn/broken sear).
- Stoppages: any interruption in functioning; must be cleared quickly.
Immediate & Remedial Action
- Immediate action: Announce "Misfire!" x3, wait 5 sec for hang fire, cycle action; repeat up to 3x before remedial action.
- For hot barrels, delay further action for 15 min if necessary.
- Remedial action includes clearing stuck or ruptured cartridges, often requiring cleaning rod or extractor tool.
Weapons Conditions & Commands
- Condition 1: Ammo on tray, bolt rear, safe.
- Condition 3: Ammo on tray, chamber empty, bolt forward, safety off.
- Condition 4: Feed tray clear, chamber empty, bolt forward, safety off.
- Commands transition weapon between these states as per procedure.
Aiming Devices
- SU-260/P MDO provides 6x magnification with BDC reticle for 100–1250 m.
- RMR sight for quick, close-range acquisition; 9 MOA dot; waterproof.
- Zeroing procedures provided for both MDO and RMR; adjustments in 0.1 mil (MDO) or 1 MOA (RMR) increments.
Key Terms & Definitions
- CLP — Cleaning, Lubricant, Protectant.
- T&E — Traverse & Elevation Mechanism for aiming adjustments.
- Hang Fire — Delay in cartridge firing after striking primer.
- Cook Off — Round fires due to barrel heat, not primer strike.
- Runaway Gun — Gun continues firing after trigger is released.
- BDC — Bullet Drop Compensator.
Action Items / Next Steps
- Practice assembly/disassembly within six minutes.
- Review and memorize immediate and remedial action procedures.
- Study zeroing procedures for MDO and RMR sights.
- Clean and inspect weapons after live fire as per protocol.