SL6/7: Expanded Roles for HK's Compact Carbines

by Peter G. Kokalis

From the end of its steel buttplate to the tip of its hooded front sight, the HK SL6/7 carbine is more than slightly reminiscent of the famous World War 11 German semiautomatic G43 rifle. But the similarity is only superficial. The G43 was gas-operated with a gas cylinder above the barrel in the same manner as the Russian Tokarev M38/40 rifles and locking was by means of hinged flaps set in recesses on each side of the receiver in imitation of the Soviet Degtyarev light machine gun.

The HK SL6/7 carbines are different. They use the delayed-blowback, roller-locking system of operation - a hallmark feature of Heckler & Koch's entire line of small artns. HK bolt mechanisms consist of two major components: a bolt head and bolt carrier. Although referred to as "locking-roller," the action is never completely locked. In the battery position, inclined surfaces on the bolt carrier lie between the two rollers on the bolt head and force these rollers into recesses in the barrel extension. After ignition, the rollers are cammed inward against the bolt caffier's inclined planes by rearward pressure on the bolt head. The bolt carrier's rearward velocity is four times that of the bolt head. After the bolt carrier has moved rearward four millimeters, the rollers on the bolt head (which has moved only one millimeter) are completely depressed, pressure has dropped to the required levels of safety, and the two parts continue their backward movement together.

A long, pivoting ejector bar is mounted to the left side of the SL6/7 receiver and is spring-loaded.

The HK SL6/7 weapons are actually a carbine-length version of the HK 630/770/940-series sporting rifles. "SL" presumably stands for Self-Loading (Selbstlade). The numbers "6" and "7" are rough approximations, in millimeters, of the overall cartridge length the action will accept. The SL6 is chambered for the 5.56mm NATO (.223 Rem.) cartridge. SOF's test rifle is an SL7, which is chambered for my favorite GPMG and sniping-rifle round, the 7.62x51mm NATO (.308 Win.) cartridge.

Compact and rugged, these slick little carbines lend themselves to applications beyond their intended roles as hunting/sporting pieces. Both the SL6 and SL7 weigh in at 8.36 pounds, empty. Their stubby 17.71-inch barrels are somewhat thicker than those of the HK 630/770/940 series. Four-groove barrels feature the hammer-forged polygonal rifling that characterizes Heckler & Koch firearms. The right-hand twist has one tum in 10.6 inches for the 5.56mm chambering and one tum in 11.0 inches on the SL7's 7.62x51mm NATO barrel. The upper and lower receivers are machined from solid bar stock.

The chambers are fluted on all Heckler & Koch and CETME delayed-blowback, roller-locking firearms. The 12 longitudinal flutes extend rearward from the baffel lead to 0.25-inch from the chamber face. These flutes reduce the cartridge case's bearing surface and permit propellent gases to flow around the case acting as a lubricant to ease extraction. As delayed blowback does not offer the available power reserve of gas operation under adverse conditions and there is no primary extraction, chamber fluting is required for reliable functioning. Ejected cases are all marked with distinctive striations but, contrary to popular belief, this does not affect their potential for reloading.

The barrels and other exterior metal parts are finished matte black. The chamber area of my specimen has a plum-colored tone often seen on Gennan WWII small arms manufactured late in the war. Although nostalgic, this finish usually signifies a hasty finishing procedure, because this color results from dipping high-nickel steel into salt-bluing solutions at improper temperatures (too low) for an insufficient period of time.

The nicely figured, uncheckered walnut stock also carries a WWII flavor in both its dull, oiled finish and configuration. The upper forearm is walnut as well and ventilated by three longitudinal 1.5-inch grooves cut into either side. The smooth wrap-around steel buttplate is retained by two large wood screws. There is no butt-trap storage compartment. A prototype green-plastic stock and forearm have been developed and will be available by mid-1985.

The front barrel band is held in place by two spfing-loaded pins of the type used on the forearms and buttstocks of all HK military small arms. These pins can be inserted from either side and I have been involved in numerous discussions as to which direction is the coffect placement. Some maintain that it is more natural for right-handed shooters to insert them from the right side of the weapon. Some insist they should be alternated. In El Salvador, troops armed with the G3 rifle insert the pins from the left side so they cannot be inadvertently pushed out when jarred against the body or combat webbing.

A fixed-fing front sling swivel is attached to the barrel band. The rear swivel is inlet into the left side of the buttstock in the European style and accepts either the web or leather HK military slings.

Cut into the stock just forward of the trigger housing is the magazine well. The staggered-row, detachable box-type magazines are of all-steel construction from the HK 630/770 series. It often takes more than a little swearing to disassemble them. Issue magazines hold four rounds in the SL6 and three rounds in the SL7, and fit flush into the stock's well. The SL7 magazine is marked ".243" and ".308," although to date the former chambering is not offered in this series. An optional 10-round magazine is available for either (the 10th round enters only with a great deal of pushing). They insert easily - but only when the bolt is retracted - and fall away freely when dropped by the magazine catch release located in the front portion of the plastic trigger guard.

Either we're all becoming more jaded or the plastic wizards are getting better at simulating metal. The polycarbonate trigger guard looks good. Two 5mm hex-head screws hold the trigger housing to the action. A screwdriver for removing them and the upper receiver's retaining screw is provided.

The drag-free, single-stage trigger breaks crisply and consistently at five pounds on my test sample. Amazing, you say? Well, this isn't your everyday G3 trigger mechanism designed to withstand a 25-meter drop test. Although the hammer is still driven by two coil springs, the auto sear has been eliminated and a spring plunger bears on the sear to yield positive engagement.

The safety lever is located on the left side of the stock, forward of the trigger guard. It cannot be reached or manipulated by the firing hand. Its spindle locks the hammer in the cocked position and it cannot be activated if the hammer has been rotated forward. To place on the safe position the lever must be moved downward until a white dot is exposed. When the safety lever is rotated counterclockwise to its upper position, the white dot is covered and a red dot becomes visible to indicate the fire position.

The folding retracting handle moves in a slot cut into the right side of the lower receiver. This is the reverse of the Heckler & Koch military small arms in which the retracting lever moves in a slot cut into the left side of a tubular extension welded to the receiver that also contains the bolt carrier's forward extension. Considerable force is required to draw the SL6/7 cocking lever rearward. A catch lever, located directly behind the cocking lever, holds the bolt group in the open position and must be depressed to release the bolt group and chamber a round. When charging the weapon, the retracting handle must be allowed to move forward without restriction. The bolt does not stay open after the last cartridge in the magazine has been fired.

The front sight is a stamped-sheet-metal disk with a rectangular post that fits into a slot in the protective hood and is held in place by a roll pin. The entire front-sight assembly has been sweated onto the barrel's muzzle.

The rear sight is the standard rotary drum found on all HK military weapons. Criticized by some, it is nevertheless a sturdy affair. There are four elevation settings and the drum can be tumed in either direction. The unmarked, 100-meter position is an open V-notch. The 200-, 300- and 400-meter positions are peep apertures, 0.062-inch in diameter. The sights are factory-zeroed for NATO standard ammunition. Use of lighter or heavier projectiles will require recalibration. Elevation zero is altered by insertion of a special tool with catch bolts into the rear-sight cylinder to mate with the cylinder's two splines. The sight cylinder is then rotated in the desired direction. This tool also contains a Phillips-head screwdriver used to loosen the lock screw and turn the windage-adjusting screw. Anyone wanting to shoot at longer ranges can replace the rear-sight assembly with the 1,200-meter rear sight used on the HK 21A1 machine gun - Why anyone would want to do this I don't know. A more sensible alteration is the knurled windage-adjustment knob offered by A.R.M.S. (Dept. SOF, 230 West Center Street, West Bridgewater, MA 02379). This knob replaces the Phillips-head windage-adjusting screw and can be installed in less than a minute. It perrnits instant horizontal-sight adjustment without a screwdriver.

Two rectangular recesses have been cut into the upper receiver to accept the HK 05 claw-type scope mount. Either 30mm rings for European scopes or 25mm rings for the standard one-inch U.S. scope tubes are available. A large vertical lever is used to compress the mount's clamps for quickly attaching or removing the mount from the rifle. While it's unorthodox in appearance, the HK 05 mount is stable and rigid. Repeated removal and re-attachment failed to affect the scope's zero. A 3mm hex wrench is provided to secure a scope to the rings.

A Swarovski ZFM 6x42mm military scope was used for SOF's test and evaluation of the SL7. This superb piece of rugged optics is available with either a plain one-inch steel tube or an aluminum tube with a NATO STANAG 2324 mount. The field of view is four degrees, or seven meters at 100 meters. Luminosity and resolution are outstanding - almost Starlight capability on a really bright night.

After initial zero adjustments, changes in elevation, from 100 to 800 meters, are made by extemal adjustment knobs calibrated for the trajectory of the 7.62x51mm NATO military cartridge. The windage-adjustment unit is located on the right side of the scope. Each calibration mark moves the point of impact one meter at 100 meters range. The reticle pattern consists of standard crosshairs in the center with thick bars at the outer perimeter. A luminescent tritium reticle will soon be available. The reticle is noncentered, i.e., as the range drum is set for longer distances, the crosshairs move toward the bottom of the field of view. While somewhat distracting at first, it does instantly alert the shooter to his range setting.

The scope is set for parallax-free viewing at 300 meters but can be changed by turning the ocular. The ocular has thoughtfully been fitted with a rubber rim to protect those who tend to "crawl" the scope. However, no scope caps are provided - an irritating omission on such high-quality optics. Six-power magnification offers the best compromise for most sniping applications. This level of milspec quality is not cheaply acquired. The ZFM 6x42mm scope carries a suggested retail price of $535. Technical brochures can be obtained from Swarovski America Ltd. (Dept. SOF, One Kennedy Drive, Cranston. RI 02920).

The SL6/7 cannot be disassembled without the aid of a 5mm hex wrench, also included. Remove the scope and magazine. Clear the weapon. Loosen the 5mm allen screw at the rear of the upper receiver - a somewhat tedious process if you take pains not to mar the stock. The recoil spring butts against the upper receiver and tension never slacks off on the screw until the very end of the ordeal. Lift up and back and remove the upper receiver. Remove the retaining screw. Withdraw the double-coil recoil spring and guide rod which run in a hole in the bolt carrier, strangely offset to the left. Lift out the rubber buffer and its sheet-metal retaining clip (not on the SL6), located at the end of the lower reciver. Retract the cocking lever. Insert the long end of the hex wrench through the retaining-screw hole in the lower receiver and compress the firing pin while lifting the bolt assembly up and out.

Stripping and reassembling a roller-locked HK bolt assembly can be frustrating the first time around. Rotate the bolt head clockwise one quarter turn and withdraw it from the so-called locking piece. Continue to rotate the locking piece and it will release from the bolt carrier along with the firing pin and spring. That's the easy part.

To reassemble the bolt group, proceed exactly in this manner. Slide the spring onto the firing pin and insert both into the bolt carrier. With its locking lug positioned upper dead-center, press the locking piece against the firing-pin spring as far as it will go and rotate counterclockwise one quarter turn. Place the beveled surface of the bolt head directly under the spring-loaded bolt-head locking lever on the bolt carrier (to do this you will have to rotate the locking piece again, ever-so-slightly counterclockwise). Slap the bolt head smartly with the palm of your hand to drive it under the locking lever. Make certain you have about 3mm of clearance between the bolt head and carrier. Rotate the bolt head counterclockwise about one half tum, until the locking rollers are at the same level as the bolt carrier's camways. Then pull the bolt head away from the carrier as far as it will go, so the rollers can recess completely into the bolt head.

The forearm can be removed by driving out the baffel-band pins and sliding the barrel band toward the muzzle. The 5mm hex wrench can be used to remove the trigger guard for disassembly of the trigger mechanism. I recommend this only for trained armorers, unless you desire to send the rifle back to HK in a paper bag along, with an embarrassed, tear-stained letter.

While there is no gas system to clean, delayed-blowback firearms become quite fouled in the area of the barrel extension immediately to the rear of the chamber face. Although difficult to reach, it should be swabbed thoroughly until clean.

Reassembly is generally in the reverse order. When inserting the bolt assembly back into the lower reciver, first place the bolt-head ejector camway onto the ejector, then push in the locking rollers and firing pin, press the assembly downward from the rear and slide it forward to the battery position. Reinsert the buffer and recoil-spring groups. Thread the retaining screw one full turn into the lower receiver. Slide on the upper receiver until its retaining slot is seated on the screw's collar. Tighten the screw firmly.

After reassembly, the headspace should be checked. This is measured by the gap between the bolt head and bolt carrier when the bolt group is in battery. The permissible tolerance is 0.005 inches to 0.020 inches. The preferred headspace gap is 0.008 inches to 0.017 inches.

One thousand rounds of military-type ammunition was selected for the test and evaluation. Two hundred fifty rounds of each of the following were fired through the SL7 fitted with the Swarovski ZFM 6x42mm scope: G.I. ball - Vietnam era - carrying a WRA '66 headstamp (Winchester Repeating Arms, 1966), PMC (also M80 configuration), FMJ match reloads (with a 175-grain, lead-cored, boattail projectile powered by 41.5 grains of Winchester Western 748 flattened-ball propellent) and West German surplus with a headstamp marked IWK 19-65 with the NATO cross in a circle.

The West German ammunition was manufactured by Industrie Werke Karlsruhe (formerly DWM) in 1965. It is packaged in 20-round boxes, which, in turn, are packed in sealed 200-round green plastic "battle paks." It is Berdan primed, but not corrosive, The lead-cored, boattail projectile weighs 145 grains. The propellant is a round-ball type with a nominal charge weight of 44 grains. This ammunition has been readily available to U.S. shooters for several years.

When chronographed, it exhibited the lowest standard deviation (22 fps) of all the ammunition used in the test. Not surprising, then, that it also produced the greatest accuracy. Under conditions of 15mph gusting winds, the IWK 19-65 shot a startling 1.0 MOA. Even though the SL7's barrel is floated in the stock, such performance through a semiautomatic carbine with a less than 18-inch barrel is an outstanding testimony to the rifle/scope/ammunition combination - a German rifle and ammo with an Austrian scope.

This was followed by the FMJ match reloads which shot 1.7 MOA. Both the PMC and WRA ammunition fell far short of this level. A mean figure of 2,700 fps is the standard NATO muzzle velocity for M80 ball ammunition. The SL7's short barrel lost only 125 to 250 fps in velocity.

There were no malfunctions except for two "bolt-over-base" stoppages induced when the weapon was purposely held away from the shoulder to permit the weapon to recoil rearward along with the bolt group. Roller-locked, delayed-blowback rifles must be held securely against the shoulder so the bolt can move sharply rearward in relation to the relatively fixed mass of the receiver body or the bolt group may not complete its backward travel and the recoil spring will not compress fully.

As it is a bit heavy for a so-called carbine, felt recoil is no more than a gentle shove rearward (also a function of the delayed-blowback method of operation). However, the handling characteristics and balance are excellent and hit probability remains high. The ejection pattem is quite forceful - about 10 to 15 feet out to the right.

Marketed as a hunting system, the SL6/7 series has a much broader potential. When properly scoped, they offer a reasonably-priced, medium-range sniping alternative to law-enforcement agencies. They would constitute a valuable addition to any survivalist battery. Their military applications remain limited by the magazine capacity and awkward location of the safety.

I can find little to criticize in SL6/7. Its outstanding accuracy potential in such a compact envelope are atuibutes which merit a close examination of this modern-day G43 look-alike. The suggested retail price of the SL6/7 carbine is $532.

First published in the June 1985 edition of Soldier of Fortune magazine