Oftentimes differentiating between certain kinds of HK parts can be very difficult because, to the untrained eyed, many of them look the same on the surface and effectively serve the same operational purpose, but, because of slight product enhancements implemented gradually over time, many similar looking HK parts can have substantially different traits when put to use. One shining example of the complexity of HK parts differences exists with the HK MP5 bolt head.
Over the course of a few decades, HK created three different types of bolt heads for the MP5. The first bolt head they developed for the MP5 had a flat face. However, because of ammo sensitivity associated with this design, this early model evolved into two later generations of bolt heads, the Action 3 and the F-Type. In the 1980s, HK added angled cuts to the face of the MP5's bolt head to better feed the new Action 3 round, thus leading to the name "Action 3" for this type of bolt head. These new angled cuts helped to improve feeding for other types of rounds as well, especially those rounds with hollow points and blunt tips.
In the 1990's, the "F" bolt head was created to address durability requirements in MP5's intended for the French police. The F-Type bolt head had a new plate roller holder added to it that is not interchangeable with other earlier bolt head plate roller holders. While the F-Type has the angled cut on the bolt face, just like the Action 3, the newer type of plate roller holder used on it is is wider, thereby making it stronger than the plate roller holder found on the Action 3 bolt head. Despite these differences, all three generations of the bolt heads developed by HK are interchangeable in their MP5s, and, today, both the Action 3 and F-Type bolt heads are still in production.
Obviously, HK is not a company that rests on its laurels so they frequently tweak and improve small features on many of their weapons over the years, thereby creating a myriad of similar items that have tiny, yet surprisingly meaningful, differences to them. And, in the case of the MP5 bolt head, HK made changes that, while certainly slight, are significant enough for people who rely on MP5s to potentially notice a difference in their weapon's capabilities depending on which type of bolt head is used.
Some examples of the these different bolt holds can be accessed via the links below:
HK MP5 9mm F-Type Bolt Head
HK MP5 9mm Action 3 Bolt Head - Stained