Hello guy curious if anyone could help me find the overall length of my new Sbr without having to remove my muzzle device.  I have a DDM4v7p with a 10.3 inch barrel with a surefire Warcomp (not permanently attached) and I’m running a BCM Mod 0-SOPMOD-Stock fully extended. I would really appreciate it if you guys could help me out or if someone with the same configuration could tell me what they put down for over all length thank you !

The form for my 11.5" barrel AR is marked 31".

I would mark it 30" and consider it close enough.

Estimate to the best of your abilities.

Bingo.

Add 19.0" to the intended bbl length.  11.5" bbl = 30.5" OAL

And round up to 30" as needed.
Getting the SBR "Tax Stamp" is a great idea.
You won't regret it.
It is great even for a 14.5" with non-pinned suppressor mount.

Awesome appreciate the help and yeah I just got my tax stamp returned that’s why I was trying to get the correct length.

So just say about 29.3 then for my length right. Okay I wasn’t sure if the aft was very strict about lengths I just got my tax stamp approved.

ATF never looks.  You can also change configuration at will.  Just make it pretty close and call it good.

You sbr the lower receiver
Just pick any 14.5” upper with a well known overall length if you need to input potential upper receiver length

Awesome sounds good thanks for all your help !

While the lower is the serialized part, that's not exactly true. You're applying for approval to make and register a firearm  -- and paying the associated tax because the configuration falls under NFA purview. Try sending a Form 1 for just a lower and you will be denied. Similarly, if you change the upper to a TItle I configuration, the lower ceases to be an SBR while in that configuration. It's only the combination of parts making an NFA-regulated firearm that requires proof of the tax paid.

While it's highly unlikely you'd ever have a problem doing what you describe, technically you're supposed to actually be making the described firearm.

Edit: OP, the last DD 10.3" SBR I stamped measured right around 28" with a Colt N1 stock extended -- from the rear of the castle nut, call it 6 1/4" with an N1 when collapsed. If you want to get semi-exact, measure your BCM stock from the rear of the castle nut and add whatever is above 6 1/4" to the 28" total and you'll be very close.

Don't be like me when I did my first and assume that the overall measured length for box 4f would be in the shortest possible configuration. The instructions state that the measured length should be with the stock extended.

Description of Firearm and Markings. (1) Item 4a. If you are modifying an existing firearm, enter the name and location of the original manufacturer. If you are creating the firearm, enter the maker's name, city and state. (2) Item 4b. The types of NFA firearms are listed in the definitions; (3) Item 4c. Specify one caliber or gauge. If there is another designation, indicate the designation in item 4h. (4) Item 4d. Show the model designation (if known). (5) Item 4e and 4f. Specify one barrel length and overall length in items 4e and 4f as applicable. Note: if the firearm has a folding or collapsible stock, the overall measurement is to be made with the stock extended. (6) Item 4g. Do not alter or modify the serial number of an existing firearm . Enter the existing serial number or, if a new firearm, one you create. (7) Markings: The maker is required to mark the firearm with the maker's name, city and state as shown in item 3b. All markings are to be in compliance with 27 CFR 478.92 and 479.102.