I am a travel agent for AAA travel and answer this type of question for a living and I guarantee you that checked luggage gets x-rayed.
It also gets ran past K-9 noses and thru the electronic "NBC sniffers"(dont know what they really call them but they are the machines that can detect chemicals thru what ever means(nuclear/chemical/biological type stuff).
Anyway, It is very normal for people to travel and buy optics(cameras..binos...ect..ect and gems/jewelry and electronics abroad. Especially, Japan for electronics CHEAP. Generally, you do have to declare it while going through customs and pay "duty" on this stuff. Have the receipt handy for sure. Otherwise they can seize it and KEEP IT.
You could contact US Customs and they would know for sure if you can buy a scope here and take it back with you but unless there is something special about it or it is catagorized differently, I would think you would have no issues at all. People do this globally every day. Diamonds and jewelry from St Thomas, Electronics from Asia, Tequilla from Mexico. Crystal from Ireland..ect...ect. Happens every day
Jamie
Jamie