I find it somewhat odd that they have trouble communicating. English is taught from grade-school on (mandatory in their standard curriculum education system, and tested for graduation), and since they are in the US they should have a greater than average ability with english because they passed their college entrance exams.
I would give them a standard english-language safety pamphlet a couple days ahead of time and let them go over it on their own. Before you go to the range, make sure that they read it and can demonstrate safe behavior with pantomime at the least.
They will be able to understand you better then the other way around, at least.
Giving them a website will also help out.
Most Japanese will only have seen a real gun on television or on the hip of a police officer. Airsoft is a very, very big there among younger adults, and it's likely that any of the students interested in shooting with you will have played with a couple airsofts at one point or another.
Civil weapon restrictions pre-date the current government, and can be definitively placed at the end of the Imperial Court era and the beginning of the period of rule by the Samurai class, which lasted several hundred years up until the Meiji Restoration at the end of the 19th century.
Peasants and merchants were not allowed to own swords and armor (though many did, looted from battlefields and such), and firearms were popular for only a short period of time during the feudal period. After the Restoration imported arms became available to people again, though largely for subsistence and criminal use. After the occupation, even sporting arms were quite rare among the populace, and there would have been little opposition to near complete bans as exist today.
Extremely strict arms restrictions are boringly common in Asian countries (heck, most countries for that matter; there are fewer free nations than we think). Legal private ownership of arms seems to be a completely Western trend.