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The construction of the GSG-5 is not identical to the MP5. The GSG-5 comprises two aluminum upper receiver halves which are joined together with a series of screws. The bolt assembly is captured

Here the four screws that hold the two clam-shell sides of the upper receiver together are visible (three left, one top right).
inside the upper. The lower receiver is molded plastic and contains the trigger group, which is not interchangeable with MP5 trigger groups. The stock, lower receiver, and fore-end are held on with three large pins which screw together much like the rotation joint on an FN-FAL. I tried to take the bolt out and remove the lower receiver to look at the trigger housing and bolt group, but I wasn't able to do so trying the obvious things. I didn't want to unscrew the upper receiver halves, nor end up with a pile of springs and parts I couldn't easily get back together without a manual.

To save on weight, the barrel diameter is pretty thin. It is not exactly free-floated, however, it only contacts the rest of the gun near what I'll call the trunion, where the forward extent of the aluminum upper receiver is joined by three screws, and just under the front sight base. Since the barrel length must be 16 inches to avoid being classified as a Short Barreled Rifle and require a $200 tax stamp, the GSG-5 is shipped in the U.S. with a "fake" or dummy suppressor, just for looks. Standard-capacity GSG-5 magazines hold 22 rounds of .22LR and the follower can be retracted to make loading easier. The GSG-5 operates by direct blowback; the bolt is not locked in battery. The bolt locks back on an empty magazine, and the GSG-5 has a magazine disconnect safety.

The GSG-5 worked fine once lubed, in the sub-freezing Colorado weather.
Gun or Toy?
While it's not built as tough as a real MP5, the GSG-5 seems pretty solid in the hand. A .22LR rifle built for plinking and sport shooting doesn't need to be battle-tough, and I don't think that should be held against the GSG-5. While most MP5 parts will not interchange, some fore-ends may interchange. The HK MP5 claw mount will work on the GSG-5, and will be the preferred method for mounting optics, such as a 1x red dot or magnified scope, on the GSG-5.

The trigger on the GSG-5 is very similar to an MP5 trigger-- in other words, long and somewhat heavy. The rear sight is a rotating disk like on HK rifles. Along with the wide front post, this sight setup isn't great for precision.

The GSG-5 may be a good training alternative for MP5 shooters.
First Shots
How does it run? I took the GSG-5 out to a range day with some of my associates (aka competition shooting buddies); we all have some time on real MP5's. The conclusion was that it ran pretty much like the real deal, down to the "HK smack" to drop the bolt. In the first 150 rounds, we had several malfunctions. After adding some lube to the bolt group, the gun ran without problems. The fun factor is high, although the sights are pretty blocky for hitting small targets. This would be a fun rifle for rimfire speed games on steel, or even a ".22" 3-Gun division (which I don't think has been done yet, though I've gotten a few requests).

We only found a few potential problems with the GSG-5. The first was that several screws came loose, specifically: (1) the screw that holds the magazine follower together (side to side); and (2) the rear sight screw. The former comprises a very small nut and bolt, and either would be extremely easy to lose. The one on the demo gun fell into the snow and I was very lucky to be able to find the approx 0.10"-wide nut. I would recommend going over the GSG-5, make sure everything is tight, and then use removable loctite to secure the fasteners. The other small issue was that the cocking tube was peening slightly on the outside surface of the notch which locks the bolt back.

We also noticed that if we placed our weak-hand on the magazine instead of the fore-end, it would induce malfunctions. Towards the end of our testing, with the rifle well lubed, the only malfunctions we had were directly attributable to touching or putting pressure on the magazine. While it's a natural thing for AR-15 and MP-5 shooters to do-- just don't!

I've been told by the importer that the problem with peening of the cocking tube notch and the magazine nut and bolt coming loose have been corrected by the factory for subsequent rifles. This kind of responsiveness and attention to feedback bodes well for the GSG-5 and future firearms produced by German Sport Guns.