Sorry, don't mean to hide anything.
Thing is, I put my hand on an antique which might not be common in any way. Therefore I'd need to prove the original caliber, to confirm it's ''antiqueness''. Since the .38s&w is on the no-go list, I'd need the ''real'' .380
380 revolver is not the same as .38 - a .357 lead will drop right through the barrel - and that is one thing that you can do to demonstrate that the gun is not on the prohib list.
A lot of Webleys can correctly shoot the standard .358 bullets.
As stated, it's not a Webley!
380 revolver is a heeled bullet with a diameter of .375
380 S&W is a non heeled bullet with a diameter of .359
They are not the same and it is unwise to use 38 S&W unless you know for certain that the revolver is in that caliber.
It is unwise because antiques are often not marked and WAAAY to many people guess. I purchased a bulldog in what I was told was 380 revolver. I was also told that I could shoot 38 S&W in it. A 38 S&W cartridge fit the chamber perfectly. Fortunately I slugged the bore first and discovered that the bore was .345. I am sure that .359 S&W would have fired but what would have happened to my fingers or to the gun is uncertain.
There was a whole bunch of antique calibers in the 38 range and a great deal of variation from manufacturer to manufacturer. It may not be 380 revolver at all. The first thing you need to do to be safe is slug the barrel. Then you will have some idea as to what will work and what won't. This is how you figure out what caliber the gun really is.