If the M4 is registered as an SBS, then it's an SBS regardless of whether you put an 18" barrel on it. I checked this out with my class III dealer when I bought mine. I already had an full size version but was just curious. Actually, you can take it off the registry but then you would be hosed if you wanted to put the 14" barrel back on there. You're fine if you want to swap barrels like that.