I've done quite a bit of extended range shooting with Glocks on MGM 2/3 IPSC silhouettes and the advice above is good. Don't think the shots are low...they are likely close to dead on or maybe even a bit over the line of sight at 100 yards. Most of the sights on Glocks set up a very high bullet path at extended range. Other things to try are to have an observer watch the target with binoculars as you shoot. You can generally see the bullets in flight at that distance without much effort and while watching for other shooters at this distance, you often see some very odd bullet performances at this distance, with bullets doing odd curves, etc.