Having flown with guns and ammo to Germany from the USA in 2017, it is not very difficult and actually was quite easy.
Shotgun Boogie can explain in detail, but I can say even leaving Germany was very easy, just having to show your guns in a suitable locked bag to the police on duty at the airport. They were not overly concerned and only had a quick peep at ours an off we went to check in.
Coming into USA you have to have the necessary form for non immigrant aliens which can be downloaded from the ATF website.
Pretty easy and you have your own guns.