Nice to find an explanation. Cause till now I thought that handgunners (and arqebusiers) were bugged and that is why they didn't do any damage. First (and only) battle I used them was a defence. I lined up 4 or five of them on a hill behind a wall of SAP's and waited for the enemy. The AI brought some heavy cav and just couldn't decide to charge the pikes so they just stupidly rode back and forth within range. Great, I thought, as my gunners begun to fire salvo after salvo. Then after some ten or more rounds I checked the enemy and guess what no casualties inflicted.
So i just thought guns were bugged and charged the cav with them pikes with good effect. In the same situation pavise arbs. decimate entire units of heavy cav in couple of shots.