The coax is kinda cool, you do have to map it as Inom pointed out, I found that out a while ago but missed this thread. Unfortunately it is not all that useful, all the coax are small calibre and have a large "beaten pattern" when fired (bullets spray all over). At typical GV engagement ranges in AH1 they are not that useful for ranging and get knocked out when the main gun does.
Maybe in AH2, with the apparant drop in typical engagement ranges, it will be more useful.