Marines A/Company, 4th Light Armored Reconnaissance Battalion (LAR), with the ammunition for the 25mm M242 Chain Gun weapon 02/10/2003. Photo Global Security:
The 25-mm gun produces its best urban target results when fired perpendicular to the hard surface (zero obliquity). During urban combat, however, finding a covered firing position that permits low obliquity firing is unlikely, unless the streets and gaps between buildings are wide. Most shots impact the target at an angle, which normally reduces penetration. With the APDS-T round, an angle of obliquity of up to 20 degrees can actually improve breaching. The rounds tend to dislodge more wall material for each shot but do not penetrate as deeply into the structure. Reinforced concrete walls, which are 12 to 20 inches thick, present problems for the 25-mm gun when trying to create breach holes. It is relatively easy to penetrate, fracture, and clear away the concrete, but the reinforcing rods remain in place. These create a "jail window" effect by preventing entry but allowing grenades or rifle fire to be placed behind the wall. Steel reinforcing rods are normally 3/4 inch thick and 6 to 8 inches apart—there is no quick way of cutting these rods. They can be cut with demolition charges, cutting torches, or special power saws. Firing with either APDS-T or HEI-T rounds from the 25-mm gun will not always cut these rods. The 25-mm gun more easily defeats brick walls, regardless of their thickness, and the rounds produce the most spall. The 25-mm gun is devastating when fired against sandbag bunker walls. Obliquity has the least affect on the penetration of bunker walls. Bunkers with earth walls up to 36 inches thick are easily penetrated. At short ranges typical of combat in urban areas, defeating a bunker should be easy, especially if the 25-mm gun can fire at an aperture. Although the penetration achieved by the two combat rounds—armor-piercing, discarding sabot with tracer (APDS-T) and high explosive, incendiary with tracer (HEI-T)—differ slightly, both are eventually effective. However, the best target results are not achieved with either of the combat rounds. At close range against structural targets, the training round (TP-T) is significantly more effective. The TP-T round, however, has little utility when used against enemy armored vehicles. The APDS-T round penetrates urban targets by retaining its kinetic energy and blasting a small hole deep into the target. The APDS-T round gives the best effects behind the wall, and the armor-piercing core often breaks into two or three fragments, which can create multiple enemy casualties. The APDS-T needs as few as four rounds to achieve lethal results behind walls. When firing single rounds, the APDS-T round provides the greatest capability for behind-the-wall incapacitation. The APDS-T round can penetrate over 16 inches of reinforced concrete with enough energy left to cause enemy casualties. It penetrates through both sides of a wood frame or brick veneer building. Field fortifications are easily penetrated by APDS-T rounds. The APDS-T round creates a hazardous situation for exposed personnel because of the pieces of sabot that are thrown off the round. Personnel not under cover forward of the 25-mm gun's muzzle and within the danger zone could be injured or killed by these sabots, even if the penetrator passes overhead to hit the target. The danger zone extends at an angle of about 10 degrees below the muzzle level, out to at least 100 meters and about 17 degrees left and right of the muzzle. The HEI-T round does not penetrate an urban target as well as the APDS-T, but it creates the effect of stripping away a greater amount of material for each round. The HEI-T does more damage to an urban target when fired in multiple short bursts because the accumulative impact of multiple rounds is greater than the sum of individual rounds. A dual-feed system uses sprockets and extractor grooves to control rounds. A system of clutches allows either sprocket to engage and feed either an AP or HE cartridge. One Bradley ammunition box holds 70 rounds, and the other holds 230 rounds. Each box can hold either AP- or HE-type ammunition. A single 1.5-horsepower DC motor externally powers the 25-mm. The enhanced gun uses a 1.0-horsepower DC motor. For all 25-mm guns, unit maintenance replaces inoperable 1.5-horsepower DC motors with the newer 1.0-horsepower DC motors. The A3's enhanced 25-mm gun has three major components: a chrome-lined barrel, an enhanced feeder, and an enhanced receiver. The gun also has quick-detachable link covers, a larger breach assembly, longer recoil, and a triple-spring drive clutch. It also has a built-in test (BIT) for the feed-select solenoid and gun-drive motor. The feed-select solenoid and the gun-drive motor each contains a jumper wire that enables the on-vehicle BIT. The gunner can choose from three rates of fire: Single Shot - the commander or gunner can shoot about as fast as he can squeeze the trigger. Low Rate - the commander or gunner can shoot 100 rounds a minute, plus or minus 25 rounds. High Rate - the commander or gunner can shoot 200 rounds a minute, plus or minus 25 rounds. The 25-mm gun's impact on typical urban targets seems magnified if the firing is in short bursts. At close ranges, the gunner might need to shift his point of aim in a spiral pattern to ensure that the second and third bursts enlarge the hole. Even without burst fire, sustained 25-mm gunfire can defeat almost all urban targets. The 25-mm automatic gun has eight cycles of function that relate to the bolt position indicator. The BPI shows the crew where the bolt is located the whole time the gun moves through its eight cycles of function: (1)Sear or Charge Cycle. The 25-mm bolt is forward of its rearmost position. The sear engages the master link. As a safety measure, the firing pin remains uncocked. The bolt position arrow points to SEAR. (2) Feed Cycle. Linked ammunition enters the 25-mm gun feeder. The feed sprockets strip the rounds from the links and move the round into the round positioner one at a time. The round positioner pushes the round into the ready position. During the feed cycle, the bolt moves about 25-mm to the rear of the sear position. The feeder rotor rotates one-third of a turn, allowing the round positioner to place a round into the rotor. The position arrow points to FEED or to DON'T REMOVE FEEDER. Absolute hang-fire protection activates in this position. (3) Chamber or Ram Cycle. The rotor stops moving counterclockwise once the round is positioned on the bolt face. The bolt, bolt carrier, and round move forward until the round seats fully in the chamber. The position arrow points to RAM. (4) Lock Cycle. The bolt is in the full forward position and the bolt carrier is still moving forward. The cam pin moving in its channel causes the bolt to rotate counterclockwise. This allows the lugs on the bolt to engage in the recesses of the breech, making a solid lock. Before the round fires, the position arrow points at the red zone. (5) Fire Cycle. The final rotation of the bolt pushes the firing pin tang off the firing pin pawl in the track assembly. This causes the firing pin spring to push the firing pin forward. In its full forward position, the firing pin's tip protrudes 2 millimeters through the bolt face. The indicator arrow points to FIRE. Although the gun requires no gas to operate, it does use 12 mm of recoil to deactivate mechanical interlock. If the breech fails to recoil, the gun stops cycling with the bolt still locked in the breech. The position arrow points to MISFIRE. (6) Unlock Cycle. Once past the MISFIRE position, the master link and slider start to the rear. The slider pulls the bolt carrier with it. The cam pin rotates the bolt clockwise, causing it to unlock from the recesses in the breech. As the bolt unlocks, the arrow points to EXTRACT. (7) Extract Cycle. As the bolt moves to the rear, the fixed T-slot extractors remove the cartridge case from the chamber. The case remains positioned on the face of the bolt. The bolt continues to the rear until it locks in the sear position. As this happens, the position arrow moves from EXTRACT to SEAR. (8) Eject Cycle. The expended case remains in the bolt face until the gunner pulls the trigger to fire another round. When that happens, the bolt moves to the FEED position. The rotor rotates one-third of a turn, feeding a round onto the bolt face and sweeping the expended case into the feeder ejection chute. As the bolt moves forward to chamber the round, the eject arm on the bolt carrier pushes the expended case from the receiver cartridge eject port.
No comments:
Post a Comment