Drozd barrel thread size

Hi there,
I bought a barrel of ebay and when I followed the instructions, I found the new 17inch one has a slightly smaller diameter although, the thread appears identical. It falls out.

Does anyone know if the auto BB guns ahve differing barrel nuts things that hold it in place?

Do I have to machine a correct thread size onto the barrel and therefore need to shorten the shroud as well, or try to find the female component?

Thanks,

John