What does the 178 stand for, you ask? The 178 denotes that a particular barrel is equipped with a 178 mm (1:7 in) twist rate, a twist rate that is used to stabilize 62 grain SS109/M855 ammunition.
The 178 marking was ultimately added to the barrels of many of their 5.56x45mm weapons because HK originally fielded their 5.56x45mm weapons with 305 mm (1:12 in) twist rate barrels in the 1960s, a twist rate that is meant for the lighter 55 grain M193 ammunition that was most commonly used during the Vietnam War. However, to modernize their weapons and keep up with the changing times towards the end of their Cold War-era 5.56x45mm weapons’ operational lives, HK adopted the 178 upgrade in the 1980s. Because this change came in an era when the weapons that received the 1:7 twist barrels were not particularly well distributed in many markets, these barrels are now considered rare, collectible items in the United States.
Examples of parts with the 178 marking can be seen with the following links:
HK53 Barrel and Front End - 5.56 - 1x7 Twist - 178 Marked
HK53 Barrel - 5.56 - 1x7 Twist - 178 Marked