I'm just a regular guy, not a manufacturer, and I want to build a civilian-legal reproduction of a non-civilian weapon.

I want to ask the ATF to certify that it's NFA legal before I build something.

I have to send it in.

How the hell am I supposed to build it to send it in to make sure it's legal without risking breaking the law if it's determined not to be NFA legal?

Fuckin' ridiculous.

If you believe there's a chance it could be a NFA item just get the appropriate stamp before building it

Most people intending to do this kind of R&D would get a manufacturer's FFL, pay for the Class 2 SOT, then build and register the item before sending it in for classification. If it comes back as "not NFA" then the registration is simply voided, and no laws have been broken.

If it's not a machinegun, you could still do that without a 07 FFL and C2 SOT, by getting an NFA stamp for the thing you think the item might be, first, then sending it in for determination. If it's determined to not be the NFA thing, you'll get your stamp money back, eventually. Again, no laws have been broken.