ryandh the reason for this issue you are having with the rounds feeding/not feeding is most likely related to the slightly different geometry of the lower > rounds & magazines > upper receiver variations. Kinda like how you can’t just buy any AR-9 upper without some research, if the lower is designed to feed from Colt cuz it is likely to not feed reliably from Glock mags without modifications.
Geometry of the feed mechanics can make a big diff. That is also why FM Products designed an entirely new lower receiver for the Mike-107 102 (7.62x39) and states specifically in their user manual that it is not interchangeable with the Mike-15 components and vice versa, even though the receiver pivot pin placement is basically the same for both.
