There are a lot of myths out there regarding the M4 that have the potential to get people hurt. For starters, you CAN reach cook off temperature in semi-auto fire. During recent heat testing of another barrel, we fired unlined DPMS M4 barrels in semi-auto and full-auto. Starting at the same ambient temperature, the M4 barrel in semi-auto hit a temperature of 230F after one 30rd mag dump. At full auto, the M4 barrel temp was 278F after a 30rd full-auto dump.

When the barrel temp hits 600F, you face a risk of cook-off. According to military safety warnings the M4 carbine can reach cook-off temps in as few as 140 rounds fired rapidly. Use common sense and safety precautions when pushing your rifles on performance. You may be able to push past these limits without cook-off depending on many other factors; but be aware that cook-off can happen at relatively low round counts in some circumstances.

Use common sense. Below is the original USSOCCOM warning message on this subject:


GROUND PRECAUTIONARY MESSAGE
ACALA #97-03
DATE: R 051413Z NOV 96
CLASSIFICATION: UNCLASSIFIED (1nn)
SUBJECT: GROUND PRECAUTIONARY MESSAGE (GPM), 97-03 5.56MM M4A1 CARBINE

1. DISTRIBUTION: {MENU} THIS IS A GROUND PRECAUTIONARY MESSAGE THAT HAS NOT BEEN TRANSMITTED TO SUBORDINATE UNITS. SOCOM COMMANDERS WILL IMMEDIATELY RETRANSMIT THIS MESSAGE TO ALL SUBORDINATE UNITS, ACTIVITIES
OR ELEMENTS AFFECTED OR CONCERNED. RETRANSMITTAL SHALL REFERENCE THIS MESSAGE. SOCOM COMMANDERS WILL VERIFY RECEIPT WHEN RETRANSMITTING THIS MESSAGE BY SENDING AN INFO COPY OF THE RETRANSMITTAL TO DIRECTOR,
TACOM-ACALA, AMSTA-AC-ASIR, ROCK ISLAND, IL.

2. PROBLEM DISCUSSION:

A. SUMMARY OF PROBLEM: SEVERAL INCIDENTS OF COOK-OFFS, IN AND OUT OF BATTERY, AS WELL AS BURST BARRELS, HAVE OCCURRED WITH THE 5.56MM M4A1 CARBINE. THESE INCIDENTS HAVE RESULTED IN INJURIES TO WEAPON USERS. THESE INCIDENTS RESULT FROM FIRING NUMEROUS ROUNDS WITHIN A SHORT AMOUNT OF TIME WITHOUT ADEQUATE COOLING.

(1) COOK-OFFS OCCUR WHEN A LIVE ROUND IS LEFT IN THE CHAMBER OR IN CONTACT WITH THE CHAMBER OF A HOT WEAPON AND HEATS TO THE POINT THAT THE PROPELLANT IS IGNITED.

(A) SUSTAINED FIRING OF THE M16 SERIES RIFLES OR M4 SERIES CARBINES WILL RAPIDLY RAISE THE TEMPERATURE OF THE BARREL TO A CRITICAL POINT.

(B) FIRING 140 ROUNDS, RAPIDLY AND CONTINUOUSLY, WILL RAISE THE TEMPERATURE OF THE BARREL TO THE COOK-OFF POINT. AT THIS TEMPERATURE, ANY LIVE ROUND REMAINING IN THE CHAMBER FOR ANY REASON MAY COOK-OFF (DETONATE) IN AS SHORT A PERIOD AS 10 SECONDS.

(C) WHEN THE WEAPON HAS REACHED THE COOK-OFF POINT (OR TEMPERATURE) A ROUND SHOULD NOT BE LEFT IN THE CHAMBER FOR ANY LENGTHY PERIOD OF TIME. THE WEAPON SHOULD BE CLEARED AND THE BOLT LOCKED TO THE REAR TO ALLOW COOL
DOWN.

(D) SUSTAINED RATE OF FIRE FOR THE M16 SERIES RIFLES AND M4 SERIES CARBINES IS 12-15 ROUNDS PER MINUTE. THIS IS THE ACTUAL RATE OF FIRE THAT A WEAPON CAN CONTINUE TO BE FIRED FOR AN Indefinite LENGTH OF TIME WITHOUT SERIOUS OVERHEATING.

(E) THE SUSTAINED RATE OF FIRE SHOULD NEVER BE EXCEEDED EXCEPT UNDER CIRCUMSTANCES OF EXTREME URGENCY. (NOTE: A HOT WEAPON TAKES APPROXIMATELY 30 MINUTES TO COOL TO AMBIENT TEMPERATURE CONDITIONS).

(F) THE USER'S MANUAL (TM 9-1005-319-10) FOR THE M16 SERIES RIFLE AND M4/M4A1 CARBINE STATES, "THAT IF A MISFIRE OCCURS IN A HOT WEAPON, REMOVE THE ROUND FAST (WITHIN TEN SECONDS). IF THE ROUND CANNOT BE REMOVED
WITHIN TEN SECONDS, REMOVE THE MAGAZINE FROM THE WEAPON, POINT THE WEAPON IN A SAFE DIRECTION AND WAIT FOR 15 MINUTES."

(G) CAUTION SHOULD BE TAKEN BY THE USERS TO KEEP THEIR FACE AWAY FROM THE EJECTION PORT WHILE CLEARING A HOT WEAPON.

(2) COOK-OFFS OUT OF BATTERY RESULT FROM A ROUND WHICH COOKS OFF WHEN THE BOLT IS NOT LOCKED OR A ROUND WHICH COOKS OFF AS THE USER IS TRYING TO CLEAR THE WEAPON.

(3) BURST BARRELS RESULT WHEN THE WEAPONS ARE FIRED UNDER VERY EXTREME FIRING SCHEDULES AND THE BARREL TEMPERATURE EXCEEDS 1360 DEGREES FAHRENHEIT. WHEN THE BARREL REACHES THESE EXTREME TEMPERATURES, THE BARREL STEEL WEAKENS TO THE POINT THAT THE HIGH PRESSURE GASES BURST THROUGH THE SIDE OF THE BARREL APPROXIMATELY 4 INCHES IN FRONT OF THE CHAMBER. THIS CONDITION CAN RESULT IN SERIOUS INJURY.

B. PARTS, ASSEMBLY, OR COMPONENTS TO BE INSPECTED: NA

3. USER ACTIONS. {MENU}

A. TASK OR INSPECTION SUSPENSE DATE (IF APPLICABLE): N/A

B. REPORTING COMPLIANCE SUSPENSE DATE: N/A

C. INSPECTION PROCEDURES: N/A

D. CORRECTION PROCEDURES: RANGE PERSONNEL AND USERS SHOULD AVOID EXCEEDING THE MAXIMUM AND SUSTAINED RATES OF FIRE DESCRIBED IN THE USER'S MANUAL FOR THE M16 SERIES RIFLES AND M4 SERIES CARBINE. USERS SHOULD BE TRAINED ON
THE CAUSES AND HAZARDS OF COOK-OFF. BETWEEN FIRING SCHEDULES, ADEQUATE WEAPON COOLING TIME SHOULD BE ALLOWED.

Wasnt this before the new heavier SOCCOM barrels..? the brief is dated Nov 96.I havent heard of this with the newer barrels and I beleive thes happened when dumping continuously on full-auto.

This applies to normal M4 barrel profiles and not the heavy M4 profiles. The more mass you have the more you can push this limit; but the 12-15rpm sustained fire rate is a good suggestion if you plan on dumping a lot of rounds.

The key point I want to make is I have seen cook-offs happen at 600F barrel temps. It did not matter whether you got there with full-auto or semi-auto either. I have never seen a cook-off happen at less than 600F. I have seen barrels go well beyond 600F (740F) with no cook off. 600F doesn't mean you will have a cook-off; but cook-offs have happened at that temp. Likewise, I've seen a lot of M4 barrels do better than 140 rounds; but cook-offs can happen anytime after 140 rounds if the circumstances are right.

All I am saying is if you want to run your rifle hard, be aware of the risks and don't be a dumbass regarding safety.

Thanks for the info that is good helpfull stuff, I've read the same in my RRA manual

Good info!

This is the same reason why I also prefer 4150 or CMV barrels over 4140.

You do not have to be running full auto to reap the benefits of the better steel.

This is especially true if you train seriously and do a lot of rapid fire shooting like as NSR drills, Bill Drills, etc where you will be firing well above the recommended 12-15 rpm sustained fire rate.

Quoted: Good info! This is the same reason why I also prefer 4150 or CMV barrels over 4140. You do not have to be running full auto to reap the benefits of the better steel

Friggin' A right man!  Thank you Bushmaster for caring!  Next build will have a CMV Sabre Defense barrel.  You seen that Middy upper from TalonArms?  OH man is it sweet, $800 bucks sweet too.

Quoted: Quoted: Good info! This is the same reason why I also prefer 4150 or CMV barrels over 4140. You do not have to be running full auto to reap the benefits of the better steel Friggin' A right man!  Thank you Bushmaster for caring!  Next build will have a CMV Sabre Defense barrel.  You seen that Middy upper from TalonArms?  OH man is it sweet, $800 bucks sweet too.

Yup, I got a Sabre CMV mid-length barrel from TalonArms for my last build and they are awesome, you won't regret getting one!

I wish mine didn't have the M203 step down, it weakens the barrel.