Colt used to make the x39 and dropped it because of the results of leo tests. The straight feed of the 20 and top of the 30 rnd ar mag design did not do well with the fat case needing to curve like in an ak magazine. The dept I worked for tried them again about 5 years ago hoping it was a follower problem and technology had caught up. They still jammed in the trials.
The dept I work for now tried them right before I started and got the same results. They were running rock river with cproducts mags and mgi bolt and firing pin. I read the report a few years ago but can't recall the figures, I think it was around 15 jams in a 1000 round trial.
Online they claim the poor results to the combo is steel cased ammo with the ar extractor design, but the dept I work for now used win, so who knows? If you don't mind give us an update when you get it in and run it through the paces.