Here is an article explaining the overall length question on the form 1, which I was confused about when I submitted mine.
How to Measure the OAL (Overall Length ) of your SBR for your ATF Form 1 Application
by National Gun Trusts April 09, 2018
We get asked this all the time. What am I supposed write in for the overall length (OAL length) for my SBR that I can't legally build until my tax stamp comes in?
We reached out to the ATF to get an answer to this question. The National Firearms Act Division recommends that you use an estimate for the overall length of the SBR. You can take the measurement of your barrel and add it the length of your lower with your stock and get pretty close to what the SBR overall length would be when it is fully assembled, a.k.a manufactured. Note: The overall length of your SBR is with the stock fully extended and any removable muzzle devices removed.
Once you receive your tax stamp back from the ATF, you can then assemble and manufacture your SBR. After you have completed that you can then measure the true overall length of the SBR. If this measurement of your SBR is different than your estimate that you wrote on your ATF Form 1 application, you will need to submit a change of length letter to the ATF. They will then send back to you an approval letter that you will need to keep with the approved tax stamp.
My receiver has multiple calibers stamped on it. Is that acceptable on the Form 1? Am I still legal if say, I get 5.56 and .300 AAC uppers of equal size/barrel length so the sbr is dimensionally identical but different calibers?
Changing Your SBR Upper with a Different Caliber Than the Registered Lower Generator
by National Gun Trusts July 10, 2018
Changing your SBR upper with a different caliber than the registered lower is possible. Since you can't use "multi" caliber on the ATF Form 1 application anymore, many people ask us. Can I change the upper caliber on my registered [tax stamped] lower SBR? The answer is yes. There a few ways to do this. The ways we recommend to do this with our clients are below:
Add this information on your ATF Form 1 (5320.1) application: To do this you can use box 4h.Additional Description. Your gun trust information normally goes here, if you are registering the SBR with a NFA Gun Trust. But you can also add additional calibers and barrel length configurations here. The ATF recommends to use this space for that because they only want one caliber and one OAL in the boxes 4e and 4f. If you have further questions about how to fill out your ATF Form 1 (5320.1 -ATF 5320.1 Application to Make and Register a Firearm), you can check out our ATF Form 1 walk-through guide here.
Send a letter to the ATF with the new calibers and/or barrel lengths: If you already have an approved tax stamp from the ATF then you can simply send a letter to the ATF describing which uppers you are planning to use with the your SBR. When you send a letter to the ATF, send the letter in duplicate and request a letter back from the ATF confirming that received/approved the letter. What you need to include in the letter is the make, model and serial number of your lower, who it is registered to, the caliber and OAL (overall length) of the new uppers that you want to add to the SBR. Below is a Changing Your SBR Upper with a Different Caliber Than the Registered Lower Generator that you can use with the ATF to notify them of these additions.
Submitting your first tax stamp is somewhat intimidating, especially since Obama changed things.
The biggest issue for me was waiting all those months and thinking that I made a mistake on the form and I wouldn't hear back from the ATF about it and would have to wait even longer after I rectified the issue.