The HK series of weapons commonly avaiable in the US consists of the HK-91 (7.62 Nato) HK-93 (5.56 Nato) and HK-94 (9mm Nato). All these rifles share common features, namely, the locking system consists of a roller locked inertial bolt, which operates as a delayed direct blow back action. Also the chambers of all these rifles contains shallow flutes starting just ahead of the web of the case, and ending just in front of the case mouth. The supposed function of these flutes is to 1) reduce case adhesion to the chamber walls, thus preventing the rapidly moving bolt from tearing off the case head, and 2) assisting is very positive extraction even if the extractor is missing, as the trapped gasses in the flutes blows the case out of the chamber. The need for these flutes is very much in doubt in HK's smaller caliber weapons, though, like the Weatherby belt, they do function as an effective trademark.
Since these rifles are not gas actions, they are very tolerant of variations in cartridge pressure and loads, they function quite well with high intensity and low intensity loadings.
The simple action effectivly eliminates jams on extraction and ejection. The aftermarket 'port buffer' intended to correct the HK rifle's tendency to fold rifle cartridge brass in half by slamming in on the rear of the ejection port, may occasionally cause the brass to bounce back into the ejection port, causing a jam. If you are depending on this rifle to defend you life with, you probably will not be concerned with scavenging your brass anyway, and can remove the port buffer to gain the extra measure of reliability.
Jams can also be caused by soft-nose bullets deforming on loading, mangled magazines, or by an very soft grip on the gun. None of these jams are intractable, but they can happen so be forwarned.
Accessories commonly found on the HK rifle/carbine's are the Claw scope mount, running in the neigherbor hood of $300 for the HK version to $150 for clones of the HK claw mount, to $50 for a set-screwed scope mount of dubious utility and durability available from B-Square and others.
Port buffers, already mentioned, keep your fired brass in better shape. The issue Bayonette is a multi-function tool that will keep you gadgeteers happy for hours.
Bipods come in two flavors, a light bipod which slides in the front of the forend, and a heavy bipod, of which I've never seen in person.
The HK rifles have quite a following, so aftermarket parts and custom smithing is fairly available. Williams Trigger Specialties of White Heath IL (area code 217) does a great job on triggers, and there are a large number of sources for Muzzle brakes, stocks, and related items.