You can transport guns and firearms to the United States as long as you ask your airline for permission during check-in.
In general, to transport your firearms, they must be unloaded and packed in a rigid face container. There are airlines that do not allow firearms even in checked baggage and also even if authorized (except for an air marshalls on duty for instance)
For sight glasses, once separated and dismantled from the rifle you have the right to bring them with you whether in checked baggage or in the cabin.
Before your flight to the United States, it is recommended to read your airline’s instructions and be well informed about firearms regulations. The law on the possession of weapons varies from one State to another according to international laws in case of transit.