If the OD of the barrel is .640 and you put 5/8'24 threads on it there won't be much of a shoulder for the can to index on, but it should be enough. .015-.020 is a very small shoulder. I think 1/2-28 would be too small and leave way too thin of a wall between major ID (.451-.453) of the bore and minor OD (.465-475 guessing no book with me) of the threads. You should be able to get away with 5/8-24 and a .015 shoulder. ALWAYS have a can or thread protector on it to protect the shoulder and threads from nicks and dings.