At first, the "modelguns" where made almost completely from sheet metal, brass and zinc alloy. 

Sometime between the1960's-70's some (very few!) enterprising Japanese citizens attempted to make real guns out of their "modelguns" (an  extremely  illegal thing to do in  "gun-free" Japan!) so the Japanese government started take steps ("the firearms/swords law") to control the replica-"modelgun" industry. 

So,  the manufactures at the time apparently got together in 1977 (and fearing an government crack down on their industry) and decided to make their own changes to the "modelguns" (self regulation) i.e. the all- metal handguns would be gold, silver or chrome plated for identification purposes, a hardened piece of steel was introduced into the barrel mold and the barrel plugged with solid zinc. 

Also, the modelguns that where made of sheet metal like the  MGC mp40,sten,sterling and others that used sheet metal/steel  for construction this method of construction  was prohibited for  manufacture/sale. 

Also, the original owner cannot sell (or give away )the prohibited modelgun,  unless perhaps he turns it into the authorities. 

Of course, all of this new regulations this did not go over well with the "modelgun" collectors who where required by law to block their modelguns barrel and paint/plate their model gold/silver! and they of course liked to buy and trade their model guns. 

Also, some  collectors where and are put off by all gold/chrome plated collections! After all, the Japanese citizen is fascinated by firearms but are often prevented from owning the real thing, and realism is extremely high on the list. 

So, the manufactures decided that if they where to manufacture the "modelguns" out of abs plastic/zinc, they could still have the expected realism, stay legal  and be less expensive to manufacture!  so, the vast majority of the "modelguns" would then be made out of abs plastic with zinc alloy parts. Of course, the manufactures like Marushin, Hudson, Tanaka and others still offer all zinc modelgun handguns.

Consequently, some of these "modelguns" are quite rare both here and in Japan, so from time to time the manufactures re-release the older models.

 lately, the  Marushin corporation has re-released the mp40,(plastic/zinc) Uzi (zinc/plastic) and the M-16A1(zinc/plastic) along the " commando" version of the m16A1 (XM177e2)(zinc/plastic) as model kits! now, if only I could only import these! (note: it seems that importation of these is possible if the modelgun is in kit form and the trade marks are removed. But as always, subject to change)

I have heard that many people have had their air soft/modelgun rifle/pistol confiscated by U.S. customs. 

So, if the company that you ordered that $500 air soft/ model gun from, has not done their home-work on the trademark/muzzle marking issue...Then, in that case,  the best you could hope for, would be for your money to be refunded. Although, some Hong Kong or Japanese modelgun dealers will work with you on the importation details: trade mark removal, orange muzzle  paint etc. But as always, be careful of who you deal with!