July 28, 2010

Troubleshooting the H&K MP5

If your hometown law enforcement uses the MP5, knowing how it works may help you understand other H&K rifles too.

American police and other law-enforcement professionals are employing the H&K MP5 with increasing frequency for potentially or obviously dangerous encounters. It has replaced the shotgun in many patrol cars, the venerable lever action in prison guard towers, and is even found in the hands of U.S. Department of Agriculture rangers for varmint and predator control. A sound-suppressed version, the SD model, is sometimes fitted with telescopic sights and used as a short-range rifle to shoot out tires on suspectsí vehicles prior to a raid.

Widely used by European police, the MP5, designed in the mid-1960s, is a scaled-down version of the German Armyís standard G3 battle rifle. It was born the HK54, the numeral 5 indicating a sub-machine gun and the numeral 4 its 9mm caliber. The current nomenclature didnít come into use until the piece was adopted by the West German police and military as the Machine Pistole 5.

Today, its multiple embodiments include the MP5A1 with butt cap, the MP5A1(NT) with butt cap and Navy trigger group, the MP5A1(3RB) with butt cap and three-round-burst capability, the MP5A2 with fixed stock plus NT and 3RB variations, the MP5SFA2 semi-auto carbine with fixed stock, the MP5SFA3 with retractable stock, and the MP5N with Navy trigger group, threaded barrel, and optional stainless suppressor.

H&K MP5

Courtesy, American Gunsmith Book of the Rifle

No matter the configuration, the MP5 is a roller delayed, blow-back weapon firing from a closed bolt. Releasing the cocking handle causes a feed pawl on the bolt head to strip a round from the magazine. The bolt carries the round into the chamber until the case mouth has engaged the forward cut in the chamber and come to a stop. In other words, the MP5 headspaces on the case mouth.

The bolt head hits the rear of the barrel extension after the extractor has extended to engage the cartridge rim, and the rollers in the boltís body are pushed out by the bolt carrier and locking piece to engage roller recesses in the barrel extension. Pulling the trigger causes the sear to release the hammer, which travels forward, striking the firing pin.

As the bullet leaves the barrel, the bolt face is driven rearward while starting the rollers out of their recesses and simultaneously unlocking the locking piece. This allows the rollers to retract completely and travel back with the bolt as the extractor claw clears the spent case. The bolt then activates the ejector lever and continues back to cock the hammer. In this process, the recoil spring compresses, storing energy to move the bolt forward to strip a round from the magazine.

When full-auto operation is selected, the sear is moved out of reach of the hammerís catch notch. As a result, the hammer is held only by the catch and released by the boltís acting on the release lever. Rate of fire is 800 to 900 rounds per minute.

The number of rounds the MP5 spits out in such a short period of time means the gun should receive service at regular intervals. On suppressed models, the suppressor must be thoroughly cleaned after every 500 rounds. Suppressed or not, after every 1,000 rounds the chamber, chamber face and inside radius of the barrel extension should be cleaned with a bronze brush and all assembly groups cleaned, then lubricated.

The entire weapon, including the magazine, should be inspected after 7,500, 15,000 and 22,500 rounds. The extractor spring and the holder for the locking rollers should be replaced, the bolt gap measured and adjusted if necessary. Another complete inspection is required at 30,000 rounds, plus a replacement of the bolt head, recoil spring, nylon guide rings, rivets and any needed bolt gap adjustment made. At 37,500, 45,000 and 52,500 rounds, the gun receives the same maintenance it did at 7,500. Somewhere upward of 60,000 rounds, it should go back to the factory for an overhaul.

At this point, letís go into a bit more detail about bolt gap, the distance between the rear of the bolt head and the front of the bolt carrier when the bolt is in battery and the hammer is forward. A gap range of 0.010 inches to 0.018 inches is ideal; 0.020 inches is maximum and results in light hits or the firing pin missing primer altogether.

Bolt gap is affected by the enlargement of the roller window in the bolt head or by a lessening in the roller/locking piece shoulder angle. It can be adjusted to fall within specifications by changing the rollers, which come in five sizes. The standard roller measures 8mm; two larger rollers measure 8.04mm and 8.02mm; two smaller rollers measure 7.98mm and 7.96mm, respectively.

Gap can be increased with the larger rollers and decreased with the smaller ones. To see if such adjustment is necessary, hold the unloaded gun muzzle down, release the cocking lever, snap the trigger and insert a feeler gauge between the rear of the bolt head and the front of the bolt carrier. A slight drag felt when the gauge is removed indicates zero bolt gap conditions.

You donít need any tools to field-strip an MP5. Engage the safety, remove the magazine, pull back the cocking lever to make sure the chamber is clear, then let the lever snap forward. Remove the butt stock locking pin and slide the stock off to the rear. Swing down the trigger group, then slide the bolt assembly and recoil spring clear of the receiver by pulling the cocking lever to the rear.

Detach the forward handguard by removing its pin and either lift up or pull down on the guard, depending on which way youíre holding the gun. Remove the recoil spring rearward from its guide. Rotate the bolt head 90 degrees clockwise and detach it from the locking piece, then remove the locking piece, firing pin and firing pin spring from the bolt head carrier. (A reassembly note: Insert the firing pin, firing pin spring and locking piece into the bolt head so the lug on the locking piece is guided through the recess in the bolt carrier. The bolt head should then be rotated counterclockwise until thereís an audible click indicating proper engagement.) To remove the trigger mechanism, uncock the hammer, rotate the selective fire lever to its vertical position, and pull it out to free the housing.

Youíll note the housing is a metal stamping, open only at the top and bottom. If you intend to fully disassemble/assemble the mechanism it contains, youíre going to be doing a fair amount of tight, inside work. So, before going any further, make a detailed, careful study of the trigger mechanism and the relationships between all its parts.

Once you have them well fixed in your mind, remove the ejector spindle by pushing it from inside the housing, then lift out the ejector and spring.

Holding the trigger between two fingers, pull back on the hammer slightly and remove the hammer axle and spring. The trigger return spring can now be raked off the flange of the hammer axle. After pushing out the trigger axle, remove the sear, trigger, trigger return spring and catch assembly axle. Remove the catch assembly and, finally, the release lever. (Reassembly tip: The wheel of the catch assembly must ride on the arc in the release lever. Use a punch to align the catch assembly spring and a cranking action to properly seat the assembly.)

Reducing the front sight into its components requires a punch to depress the spring and plunger in the sightís holder. That done, rotate the cap 90 degrees, drive out the tapered clamping sleeve roll pin front to rear and remove the sight insert. The front upper edge on its blade is beveled.

A special tool is needed to remove and reinstall the rear sight assembly. Maybe you can buy one from the factory and maybe you canít. If youíre successful in securing one, insert it into the sightís drum and rotate to remove the drum. Be careful; there are detent balls and springs just waiting to fly out from under there.

With the drum out of the way, removing the Phillips head screw and washer beneath it presents no problem, but getting the detents and springs back in does. You have to restrain them with tweezers until the drum half captures them during reassembly.

The magazine release is disassembled by driving out its clamping sleeve (it looks like a pin), pushing the release button through, raking out the non-captive spring with a fingernail or small tool, and removing the contact piece. When reassembling the MP5, use a pronged screwdriver to reinstall the spring, so grind a special tool for this job.

Disassembling the cocking lever isnít tricky or tough, but youíll likely never have to take it apart unless itís broken or bent. This could occur if an MP5 were run over by a truck, but that would probably call for a whole new gun anyway. However, with the lever to the rear, move it forward in its notch to reveal the axle (pin). Being careful of your alignment, and to avoid damage to the cocking leverís housing, drive out the axle, then remove the lever, the catch bolt and catch bolt spring.