Well, I learned something a couple weekends ago. While I was explaining the product upgrades of the Gen5 line to previous generations I was asked about the rifling. I field stripped the 19.5 I was showing and grabbed a G43 from the case to highlight the differences in rifling...only to see "GMB" rifling in the 43 barrel. Thinking that *maybe* Glock was just transitioning all 43 barrels to GMB to ease 43/43X production I grabbed a 42 from the case. It also has GMB style rifling. Having no Gen3 or Gen4 9mm guns in stock I used an HK P30 barrel to highlight the traditional polygonal pattern.
So this morning I just remembered to check my 2015 vintage G43 and it also has GMB style rifling. As well as my 2014 G42. To make sure I hadn't smoked crack (and forgot about it) I compared my G43 barrel to my G43X barrel - rifling pattern is identical.
Now I feel really dumb for not noticing this in 5.5 years, the G42 I should have picked up on back in 2014. Given the real-world results in precision, I hazard that this rifling design was developed to ensure that the 42 and 43 met ATF accuracy requirements.
So is this something that was known and I was just in a different dimension?