Preparing for my first EF1 SBR and this is what I have gathered and understand

The overall length is with the stock extended

The OAL is measured from the outermost part of the butt stock to the end of the barrel without a muzzle device.

I will be SBRing one of my LMTs.  I am using numbers below as an example not actual numbers:

If the OAL of my LMT is 33", 16" barrel, no muzzle device, stock extended.  And I will be installing a 10.5" barrel from LMT, I don't have the barrel yet, will wait to get approval before purchasing, since the 10.5" is 5.5" shorter than a 16" barrel, I will list the OAL on the EF1 as 33 - 5.5 = 27.5"

Does this sound like the right approach or am I overthinking it

77

Add 19" to barrel length

Your barrel length math seems right. I’ve also used the 19” plus barrel length estimate and it’s within an inch and good enough to fill out a form on a modular system before all the parts come together.

If you have a lower that was a pistol build first you can have a short upper before waiting on a stamp.  If you are building the upper, you can buy the barrel in advance and wait to install it (an SBR isn’t an SBR unless it’s configured as one).  Even if none of those conditions exist you can buy the short parts and sequester them somewhere outside of your immediate control and ability to configure an SBR. It sucks to wait on a stamp and find parts have dried up, out of stock, AR57 stopped building, etc.  You can always sell unused parts.

What I have is a LMT rifle, purchase as a factory rifle. Will SBR this one.  I already have a SBR so I can get the barrel up front and still be legal.

Just doing some planning/thinking ahead

This is what I have also done.

But per the OP's math, his is only 17" fully extended.  Is that right OP?  What stock are you running on the LMT?

The numbers I used in my original post were an example,  not actual numbers.

I will work on actual numbers....opened an account on the ATF website last night.  Need to get all of my data together before I start the process.

77

If you are using a trust having a scan of your trust and a scan of your signed RPQ as well as scanned or electronic passport photos ready to go is handy.  In my experience the longer you have a session open, the more likely eForms will crash. Scanned docs much smaller than their 3Mb limit is also useful.

I figured as much as my OAL with a 8.5" bbl is ~27.7"

good luck with the application.