Brother SRM wrote:They became strength 4 with the Dark Angels codex, which was actually near the end of 4th ed. Gav Thorpe wrote that one.
Ah, I guess the 5E rulebook just wanted to list them both - the manstopper round for Dark Angels / Blood Angels and the standard shell for the other Chapters. A short internet search on the subject yielded a few old discussions about this topic, as well as that there was apparently even a fluff blurb that explained why the
DA used more powerful shotgun rounds. Back then they were described as unique to them, but after the
BA got them as well, a number of players speculated that this is a trend that would soon catch up with the other Chapters, too. Turns out they were right.
As a sidenote: said manstopper rounds are also available for normal people (well, gangers and Arbites) in Necromunda games, where they push shotgun damage from S3 to S4 as well, so it's not exactly a Marines-only thing. I guess it's just not standard issue for the Guard, being more expensive and all.
Mystery solved, thanks.