For the pistols i find that i'll just reload after every dance, 15 rounds is usually enough to put someone down, if your not lucky enough to pop one in the head, if 15 rds cant do it, your prolly outgunned anyhow. I'm not really a big fan of the 92fs so if it comes down to that i run for cover, and ask for help.
As for the M16 and MP5. Both have 30 round clips. Simply take note of your total ammo count when your sure you have a full clip in weapon. So if you pick up one with 55rds reload it if possible, and then you know that round #26 is the last round in that clip.
Personally i'd rather see a percentage bar with a lable for that clip's full ammo potential. That way i wouldn't be able to know the exact number of rounds in the clip.
Either that or have icons representing your clips, and then have them dim out as the ammo is spent. Only problem with that is there would have to be some way of tracking how much ammo is in each clip you have. You wouldn't be able to fire 3 bursts reload, fire 3 bursts reload, fire 3 bursts reload, and always have a full clip if you needed to go full auto as i always do. heheheh.