I don't know yet and am thinking through it - you have to read through the enforcement letter and the part on similarity and interchangeability - words like substantially different are used - similarity and interchangeability refer to comparisons to the enumerated weapons (9 in total) as part of the AWB ban
It could be with the 9mm bolt, different maxwell, different barrel, etc that it falls outside of the new definition for an AW
Up to your interpretation - I have a 9mm SBR build I was planning to do and need to think through this as well