I'm pretty sure the GEW 88 is classified as an antique (it's dated 1890 and still the 8mm bore diameter). But what about an 1889 Mauser,dated 1916 and made in Birmingham, England? Can it be shipped to other individuals as an antique?

Manufacture before 1899 is an antique regardless of a model number.

So all GEW 88s are antiques since they were all manufactured before 1899.

Some 1889 Mausers would be antiques and some would not depending on actual date of manufacture.

Kinda nutty huh?

Definition of an antique

If it's originally dated (manufactured) after 1898 then it's not an antique. If the date of manufacture (NOT the model number) is 1898 or before, then it is an antique. Your 1889 (model #) was apparently manufactured in 1916 (date of manufacture or possibly rework) and so it is considered to be modern.

As an example, I have several Swedish Mausers and 2 are dated 1895. These are considered to be antiques although an identical rifle dated after 1898 is considered modern. Ditto anything made or sold by Ludwig Loewe of Berlin. If it's marked Loewe then it's an antique since Loewe became DWM in 1896 and so anything made or sold by them necessarily HAD to be made pre-98. Kinda confusing in some ways, good luck.
Regards, Joe