I shoot at a rifle range that's set up in metres, not yards. How will this affect me when sighting in a rifle? If a scope is dead on at 100 yards, it won't be at 100 metres.
The difference between metres and yards has no effect when sighting in a rifle. As long as you know the ballistics of your cartridge/bullet combination, you can sight in at any distance you choose. For all practical purposes with most hunting rounds, a zero at 100 yards versus a zero at 100 metres would have little differential to worry about in terms of downrange performance. If you want to be exact, however, I would recommend using one of the many online ballistic performance calculators and plugging in your actual data, including the range at which you're zeroing your round. These calculators will provide ballistic performance measures out to any distance you select.