The Full Test
Army TM9-1005-313-10, the operator’s manual for the M240 series, calls for the following rates of fire:
Sustained: 100 RDS/M (4-5 seconds between burst) – Change barrel every 10 minutes.
Rapid: 200 RDS/M (2-3 seconds between burst) – Change barrel every 2 minutes.
What this operator manual describes is not a Mil-Spec “test,” it’s simply the recommendation for the user for his barrel change times. Additionally, there is the “Hasty Defense” test previously mentioned.
The Main Test- 50,000 rounds through one U.S. Ordnance MAG-58
The real benchmark to reach for is reliability. In U.S. government test wording: The machine gun shall demonstrate a Mean Round Between Stoppage (MRBS) of 7,500 rounds and a Mean Round Between Failure (MRBF) of 25,000 rounds over a minimum receiver service life of 50,000 rounds. Barrel life shall be 15,000 rounds minimum.
In order to properly do the testing, a series of firings at set intervals were to be done with one machine gun. This machine gun would have to undergo the grueling test until it reached 50,000 rounds, and the testers would have to rigidly observe firing, cleaning, and parts replacement requirements from the test protocol. Grueling on the testers as well…
Once the testing started, the protocol had to be observed. Barrel temperature had to be kept within certain ranges, and burst firing had to be kept to the details of the test.
The test protocol was the same as used for the U.S. military with some modifications due to the weather:
- One machine gun with Mil-Spec 7.62x51mm ammunition.
- 3,000 rounds per session with infrared thermometer checking the barrel temperature to stay within the protocol.
- 10 minutes of 100 rounds per minute, fired in 5 round bursts with equal time between bursts.
- 5 minutes of 200 rounds per minute, fired in 10 round bursts with equal time between bursts.
- 5 minutes of 100 rounds per minute, fired in 5 round bursts with equal time between bursts.
- 3,000 rounds now fired- cleaning in the field and parts inspection.
- Repeat 3,000 round cycle.
- 6,000 rounds- thorough cleaning and full inspection and gaging at factory.
- Repeat 6,000 round cycle and return to factory for full inspection and gaging.
- 7 cycles of 6,000 rounds were performed in proper cadence, then the last section added 2,000 rounds more at 100 rounds per minute and the test of 50,000 rounds on one machine gun was finished.
Mean Rounds Between Stoppages (MRBS) is determined by dividing the total number of rounds fired by the total number of stoppages. “A stoppage is defined as any unplanned cessation in firing or the inability to commence or cease firing attributable to the gun. All incidents shall be recorded and any considered as not chargeable to the machine gun shall be substantiated by the contractor.” During testing we had (3) total stoppages. Two were properly attributed to the ammunition and (1) was a light primer strike. Ammunition issues do not count in this type of test, but the light primer strike was considered a stoppage. The math on that is: 50,000/1=25,000. That’s “25,000 Mean Rounds Between Stoppages.”
Result? The U.S. Ordnance M240 system outperformed the U.S. Specification of 7,500 Mean Rounds Between Stoppages by a factor of 3.
Mean Rounds Between Failures (MRBF). MRBF is determined by dividing the total number of rounds fired by the total number of failures. “A failure is defined as any stoppage which involves part replacement or requires in excess of one minute to correct; or involves any failed or damaged part detected during scheduled preventive maintenance, the replacement of which is not authorized at the crew or organizational level of maintenance as prescribed by Source Maintenance Recoverability Code and TM-9-1005-313-23P.”
During the 2nd day of testing we replaced the drive spring during the regular scheduled maintenance, as allowed under the test protocol. On the 3rd day during testing we encountered a failure due to the bolt roller detaching. This item was replaced and the system performed well until the conclusion of the 50,000 round test. The math on that is: 50,000/1=25,000. That’s “25,000 Mean Rounds Between Failures.”
Result? The U.S. Ordnance system matched the aggressive U.S. Army specification of 25,000 MRBF.
