Of all pistols, HK's are the worst. First of all, there are a number of different sites on each model (especially the P7), so buying something aftermarket can be a real challenge. Then, you need a very expensive special tool to remove and replace them. If you don't use the tool, you run the risk of damaging the slide. If you have an HK pistol, send it to an HK smith. Let them site it in a machine rest- especially if they're non-adjustables.