I know this is pretty noob-ish, but for me, the subject is as clear as mud and I'd like to get it straight.

I get the whole Form 1 thing for a "typical" firearm (I did a paper Form 1 for my SBR), but building a suppressor is sort of WAY different.  I understand how to be specific, how to state overall length, caliber and so on, and I probably also get the marking/serial number part.  What goes in the "barrel length" section?  What other description is needed in 4h - diameter, design, other?  What else am I missing?

I would be using my trust and using e-file, but it all relates back to the paper form, so I want to have a really good idea what I'm doing before I get started...

http://www.ar15.com/forums/t_6_17/415446_Visual_guide__How_to_fill_out_a_Form_1_using_EFORMS.html

The two things worth noting is that for a Form 1 can you list the max caliber and it us just the number.  If you building a suppressor that would work with a .308x51, 7.62x39, .223 etc you would list it as 308 or 7.62  Leave off the preceeding (.) and (x39).  You don't have to have a model name for a Form 1 Suppressor, I would leave it blank, otherwise you would have to engrave that.  Approval time is around 25-35 days now.

Follow the link in the previous post.  Don't put anything in 4h.  The OAL is the estimated length of suppressor you intend to make.  Over estimate this dimension, as it's perfectly legal to make it shorter, but not so much making it longer.  eForms gives you the ability to attach documents in the "Item" section, and I just attach a word document clarifying that the application is for a silencer that will be made on behalf of the trust.  You need this because when you submit the form, it will immediately go into a pending research status, and this attachment will clear up any issues ATF may have regarding the manufacturer code you entered.  Caliber requirements are the same as if you were using eForms for making a SBR...

4h stays blank.
Barrel length is N/A.

So it looks like it's way simpler than I was afraid it might be.  Form spaces that don't apply are N/A, and I need to give a maximum length but can go lower if I need to.  Otherwise it's pretty close to a Form 1 for an SBR.  Am I on track here?

You've pretty much got it...

Great.  Thanks for helping me out, folks!

As far as the N/A areas, I just leave them blank...