Sources indicate that the SMG II was designed in 1984 for a classified end-user who was well familiar with the advantages and shortcomings of the entire MP5-series of weapons. The result of HK's development and user feedback was a synthesis of some of the more desirable features within the MP5 system, several features from other weapons such as the UZI, and some new developments unique to the SMG II.
At a glance, the compacted SMG II bears a striking resemblence to the MP5K. The gun is equipped with a veritcal foregrip, the barrel is shrouded and does not protrude beyond the front sight, and nothing overtly extends from the rear of the double push-pinned receiver cap.
The sight profile is nearly identical; the front sight is a hooded-post affair, and the rear sight is the familiar rotating-drum type. However, a closer look at the SMG II reveals that even these similar looking features differ in varying degrees from those of the MP5K.
Starting with the sights, the rear rotary aperture is similar to the peep type that is found on the MP5, rather than the open notch type typically found on the MP5K. Two mounting brackets which are spaced roughly 3 inches apart lie along the top of the upper receiver and allow the attachment of various types of optics. These brackets are unique in two ways: 1) they differ in shape from those found on other HK submachine guns and accept mounting devices similar to those used on the MSG90, G41, or 21/23 while 2) they are positioned further rearward than on other MP5-series weapons, the rear bracket lying directly against the rear sight. This is significant because it leaves about 5 inches of space between the front mounting bracket and the front sight - an area in which the weapon's cocking handle is positioned to allow ambidextrous use like the cocking handle on an UZI or a MAC-10.
At the rear of the weapon, what looks like an MP5K's receiver cap is acually the buttplate of a streamlined collabsible stock. Like that found on any A3 variant of the MP5, the stock's rails slide along channels in the upper receiver. But unlike the MP5, the SMG II upper receiver's stock channels are located directly above the ejection port. The stock rails are still shaped such that they fold downward along the contours of the upper receiver, and the right rail would obscure the ejection port if it was not properly machined in that area. On either side of the stock's base are ambidextrous sling mounts (there are also similar mounts on the front sight). The stock's release button is located on the top of the buttcap just aft of the rear sight. It is questionable whether or not it can be deployed faster than the collapsible or folding stocks found on the MP5 or MP5K. But the SMG II seems better balanced, and nothing protrudes downward from the rear to inhibit firing the weapon like a pistol.
Both of the SMG II's pistol grips are shaped like those found on the Navy trigger housings - perhaps this is no accident. Anyway, for those who are not familiar with the variations among MP5 trigger housings, the Navy versions lack finger grooves and gradually flare outward towards the bottom, where a small lip serves to keep the hand from slipping downward. Both of the grips are hollowed and facilitate the storage of either a cleaning kit or spare parts kit. There is a small lip at the front of the foregrip which keeps fingers from wandering along the bullet's path. Said lip is not as exaggerated as that found on the MP5K; consequently, the foregrip can be positioned farther forward of the magazine well than on the MP5K. This leaves ample space between the magazine well and the foregrip to allow the operator the option of holding the SMG II as one would hold a standard MP5 - with the forend rested across the palm of the hand. At the rear of the forend and adjacent to the gas valve lever are white markings indicating the two valve settings (more on this feature below). Between the forend and the magazine well lies the lower receiver's push/hinge pin.
By now, some astute readers may be wondering why mention has been made to the SMG II's lower receiver. The answer is simple: unlike any of the MP5s, the SMG II actually has one. This lower receiver outwardly represents the SMG II's departure from the scaled-down-G3 characteristics of the MP5 family of weapons. It houses a fire control mechanism along with the magazine well, magazine release, and bolt lock button. Each of these devices in turn differ from those found on the MP5, especially the bolt lock button which actuates a feature alien to the MP5 or other HK rifles.
A familiar-looking feature of the lower receiver is the fire control mechanism. The ambidextrous selector lever is ergonically identical to that used on the MP5 Navy trigger housing. It can be rotated to one of four positions : 0, 1, 3, or 30. Each number indicates the maximum number of rounds fired with one pull of the trigger, with 0 representing the 'safe' position. Naturally, one round in the magazine makes the 3- or 30-round position sort of a moot point, and it is assumed that the SMG II's polymer magazine holds no more than 30 rounds.
The trigger area is more elongated than on the MP5; the trigger guard advances into the region behind the magazine well that is normally occupied by the MP5's magazine release. This is possible because the SMG II's magazine release is positioned directly above the trigger area - part of a two-phased approach to making the release more user friendly. If MP5 users tend to have any complaints about their own weapons in this department, they are as follows: 1) the magazine release button is too far foward to be reached by the trigger finger, requiring use of the free hand to disengage it and 2) right-handed operators are at a disadvantage because it is on the right side of the weapon. HK has managed to work around these problems inherent to their rifles and submachine guns by the addition of a flapper mag release. However, they designed the SMG II with some consideration here so that either right- or left-handed operators could reach up and press the ambidextrous magazine release with their trigger finger. This may seem like a long-winded explaination to something rather trivial, but the speed of a magazine change is directly proportional to the chance of being stitched by an adversary in the process.
A second ambidextrous button located just above the magazine release actuates the bolt lock. When used in conjunction with a suppressor, this feature reduces the weapon's sound signature by locking the bolt closed. The SMG II is not the only firearm HK ever fielded with a similar device, though. Prototypes of the now-familiar MK23, or SOCOM pistol, had a bolt lock button inside the trigger guard just above the Laser Aiming Module's mounting stud. Obviously, employing such a device requires the user to manually cycle the action for follow-up shots. Operators sans discretion may question the trade-off here, but a bolt lock is arguably a worthwhile option.
Another feature that enhances the SMG II's stealth as well as its reliability is a forward assist. It is equipped with one that is more streamlined than that of the G41 or PSG1 and is situated between the stock rail and the selector lever. Southpaws can operate this assist with their thumbs, while right-handed users must alter their grip a bit more. Like the forward assists commonly found on rifles, it allows the user to chamber a round when the weapon is excessively fouled. But perhaps more importantly, it facilitates the quiet chambering of a round when the user does not wish to let the charging handle slam forward.
While a forward assist serves to bolster the element of surprise, covert operators tend to rely on suppressors to maintain stealth when the weapon system allows. All of HK's submachine guns except for a few variants of the MP5K either allow for the attachment of suppressors or, as in the case of the MP5-SD, come with integrated suppressors. When HK's engineers (armed with no small measure of user feedback) developed the SMG II, they bestowed upon it a number of the advantages associated with each of their previous designs. The SMG II's free-floating barrel is threaded approximately 3 1/2 inches from the muzzle so that the suppressor's mount slips over it, thereby eliminating the need for either lugs or a barrel that protrudes beyond the front sight. The barrel shroud is ported so that the user can visually ensure that the suppressor is mounted properly. Also, the SMG II is equipped with a gas-bleed valve which allows the operator to reduce the speed of supersonic bullets to subsonic velocities. This valve is controled by a lever that rests just above the lower receiver's front hinge pin, and it can be rotated to one of two positions: open or closed. One advantage that a system such as this has over the MP5-SD is that the user maintains the option of reducing the bullet's velocity, terminal performance, and report.
HK reportedly has handmade at least 60 SMG IIs for one or more classified clients, and this is not surprising for several reasons. First of all, they have been known to custom-make firearms in the past - namely specially engraved or plated weapons for Middle-Eastern royalty. Also, their engineers have proved quite willing to take user feedback directly to the drawing board. Evidence of this can be seen in the MP5K, which was supposedly designed at the request of their South American sales representative, and in the addition of user-inspired features such as a bolt catch to some of their existing weapons. The SMG II represents the logical evolution of HK's proven MP5 which, by 1984, had already come to be recognized as the pre-eminent submachine gun by many within the free-world's law enforcement and SpecWar communities. However, in today's free world, the SMG II's versatility allows its user enough options to guarantee that it will never be placed in the hands of any other than a select few.