You can "do" whatever you want. However if FRB looks at it and determines that it is strictly a lower, they will throw it out and not scan it into the system. That is the word I got from those in charge.
If you lie and buffalo them it will go into the system. Just keep in mind that you fill those things out and sign them alleging them to be true and if found to perjure yourself (and they decide to do something about it) you can kiss your LTC and guns all goodbye in MA forever.
My question is - why would you want to? If you are building an AR-15, wait until it is capable of firing a bullet, then send in the paperwork.
Why give them anything more than is strictly required? Not that there's any real clarity in what is required, but the FA-10 stuff is clearer than most. I wait until day 6 before I mail my FA-10s.
All this talk about another AWB got me worried. I bought the lower the other day trying to get it in before they can ban it. I thought they did a fa-10 when they did the nics check. I wasnt planning on building it for a long while