The antique classification has nothing to do with caliber, it has to do with when the gun was made.

The GCA Defines an antiques firearm as one made in or before 1898.

If you can prove the gun was made in or before 1899 you can present the following statement to Customs when you clear the gun:

To whom it may concern,

The enclosed firearm was manufactured in or prior to 1898 and meets the definition of an antique firearm under Title 18 Chapter 44 section 921 (a) (16) of the Gun Control Act of 1968. For the purposes of importation under Part 47, this firearm is defined as not a firearm in Subpart B section 47.11. Part 178 Subpart G 178.115 makes the importation of this firearm an exempt importation. No ATF Form 6 import license is required.

This firearm is being imported for personal use and is not for resale. This firearm is exempt from duties as an Antique under HTS 9706.0000.60.

Of course this is not to say Custom will clear the gun before confirming with ATF.

You better be able to provide proof the gun was made in or before 1898.

The best way to avoid problems with Custom these days is to submit an ATF form 6 and a cover letter asking ATF to confirm the gun is an antique. Take the reply letter from ATF with you to Customs. They may call to verify the letter but you should not have any problem clearing the gun with the ATF letter.