SAR Tests the FN 240-B 7.62mm Machine Gun
By Dan Shea
SAR traveled to the FNMI facility in Columbia SC for the “Cook’s tour” of the facility, and the chance to root through the parts pile upstairs while testing out the M249, M240 series, P90 and FiveseveN pistol. George Kontis and Sal Fanelli from FNMI were most helpful, allowing SAR to shoot as much ammo as we wanted. Well, almost. The Mid Carolina Rifle Club graciously allowed us to use their range for the testing. The following article is the first in a series of specific articles on FNMI’s products.
The United States Military’s search for a replacement for the venerable M60 GPMG has been officially over for a short time. We went from the Browning series of .30 caliber guns, to the “Throw-away” .30 caliber M60, added the 5.56 caliber Minimi series, then got serious about the General Purpose Machine Gun role again with the M240 MAG series. Your faithful correspondent has always been a fan of the M60- but that must be kept in context- the M60 was designed as a 100,000 round gun; sheet metal receiver components, issued with a receiver stretch gauge. Door gunners burning 5,000 or 6,000 rounds a day ate that 100,000 rounds up real fast, and tossed the guns out for a new one fairly regularly. In contrast, it is not unusual to hear MAG series gunners speak of “5 million round” MAGs. “5 million round” guns! The M240 series guns in use by the US Military have a rating of 26,000 MRBF. That is “Mean Rounds Between Failures”. On average, the M240 series guns just don’t have problems.
While the weight is significantly more than that of the M60, the robust construction has added to the reliability of the gun. This article is not on the MAG or M240 series weapons- it is about the changes made for the newly adopted M240B model that the US Army has specified and purchased.
The basic changes from the M240G are as follows; the addition of a dust cover over the ejection port, heat shielding hand guards, single rate of fire (750 rpm), and an integrated optical feed cover which allows for scope mounting.
To understand the M240B, it is necessary to take a closer look at the FN MAG-58 series of weapons and how they function. Many times these guns are referred to as “Upside down BAR’s”, and there is some truth in that simile. However, for SAR’s purposes today, we are going to take you through a cycle of the weapon instead of going into the historical roots of the design.
One Cycle:
For purposes of our sanity, we are going to refer to the Piston Assembly as the “Op rod” and the Breech Block as the “Bolt”.
The weapon is properly prepared and maintained, top cover opened, and a belt is placed in the feed tray. (The 7.62 x 51 mm ammunition is linked in either M13 disintegrating links, or the DM1 belt. M13’s are standard M60 GPMG links, DM1 belts are the same as the HK21 uses). The charging handle is pulled to the rear, then returned forward. This leaves the Op rod held to the rear by the sear notch. The recoil spring is compressed.
The trigger is depressed, lowering the sear. The recoil spring drives the Op rod assembly forward, and the feed horns on the bolt contact the lower edge of the cartridge base, moving the cartridge forward out of it’s link. The nose of the bullet is guided down into the chamber on the feed ramp, and the cartridge heads into the chamber.
Before the cartridge is chambered, the concave part of the locking lever contacts the ends of the main front guides on the sides of the body, causing the locking lever to come down into the lock position. The cartridge now reaches the full chamber position, and has the bolt locked solidly behind it. (The extractor has positioned itself over the rim of the cartridge as the cartridge “Bottoms out” in the chamber). Forward motion of the bolt has stopped.
The Op rod continues forward and drives the firing pin into the primer. Forward motion of the Op rod is stopped when the forward face is seated on the gas cylinder. It should be obvious by now that the firing pin can not go forward until the bolt is firmly locked against the lugs on the receiver.
Once the cartridge has been fired (We are not going into the physics of the expanding propellant gases), the bullet proceeds down the barrel. As it passes the gas vent on the forward section of the barrel, gas is bled off into the gas regulator, then into the gas cylinder. This system is pressurized for the time remaining that the bullet is in the barrel until it exits the muzzle. The pressure is relieved as it drives the Op rod backwards.
The Op rod’s backward motion relieves the pressure on the firing pin, and activates the link, pulling the locking lever up away from the locking lugs. This cams the bolt to the rear at a much slower rate than would be expected, allowing the extractor to reliably start pulling the cartridge out of the chamber. This slow rate makes for a very reliable extraction process. It is referred to as “Primary extraction” and is one of the major selling features of the MAG series of machine guns.
As the Op rod continues it’s rearward motion, the bolt is completely unlocked and the mass of the Op rod and bolt continue to recoil as one. The cartridge base, held by the extractor, strikes the ejector and the spent case exits the receiver to the bottom. The Op rod assembly continues to the rear, compressing the recoil spring, then strikes the buffer in the buttstock face.
At this point, if the trigger is still depressed, the cycle repeats itself, or if the trigger has been released, the sear catches in the sear notch on the Op rod and the weapon is ready to fire at will.
To understand the M240B, it is necessary to take a closer look at the FN MAG-58 series of weapons and how they function. Many times these guns are referred to as “Upside down BAR’s”, and there is some truth in that simile. However, for SAR’s purposes today, we are going to take you through a cycle of the weapon instead of going into the historical roots of the design.
This article first appeared in Small Arms Review V1N6 (March 1998) |
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