The front sight, gas block, and front trunnion (one at a time) are pressed into position on the barrel. These are all very hard, so the holes can be used as drill guides. Be very careful and go slow. These were all pretty easy. The biggest problem I had was the drilling the angled gas block hole into the virgin barrel. If you have problems with this, don't force the issue. I recommend if if it is difficult, remove the gas block from the barrel and use the mark made by the drill bit as a guide for location. It would be a tragedy to have a broken drill inside your gas block. Don't worry, with time and patience it will come together well. Have a good time with your build!