Can someone explain the process of going about buying a SBR upper? Also, what is the most common SBR length for 5.56? 11.5”?

In a nutshell:  Locate your upper and pay your money. SBR upper ships from manufacturer to your CLIII dealer on a Form 3. Once in the dealer’s hands, submit Form 1 to transfer to you.  Wait 8-9-10 months to receive approval, then pick up your toy. Engrave your lower and add the upper. Rounds complete.

Popular sizes are 10.3”, 11.5” and 14”.  Everyone has their favorite for different reasons.

Stand by for others with more/better information, and some great suggestions!

Upper is easy, it's the lower that counts. It is the registered receiver.

This, you can order an upper and have it sent to your house.
The lower is the part (if it is an sbr lower) that you need to go to an ffl and file NFA paperwork

Not sure what ^^^^ he is talking about??
You can do an eform 1 and convert your existing AR15 lower into a SBR lower for $200. After approval you need to engrave it with the new "maker" of SBR information such as Individual, trust, LLC etc. I have done 9 of them and the longest has taken me 23 days. Shortest 16 days for approval.
You can do an eform form 4 and buy an existing SBR from a FFL/SOT/Manufacturer and wait 6-12 months for an approval that also costs $200 for tax stamp. No engraving needed

There is the Mk 18 which is 10.3", There are several 11.5", 12.5" and such uppers that you can just buy and they ship it to you. Just don't assemble it on your lower until it is approved and you have the $200 tax stamp.
I have 10.3", 11.5", 12.5" and 12.7" uppers

https://www.ar15.com/forums/Armory/Short-Barreled-Rifles-SBR-/51/


It's all in there.

Ar15 uppers are not the registered part of the rifle therfore there is no such thing as an sbr upper

Yes - that's the first step - understand the product. Also, an "SBR Upper" is often seen as a Pistol Upper, and are not a regulated part. (in most states) The lower is the part that you need to be concerned about. It's the actual "firearm" and the part that has a defined status of Receiver, Pistol, Rifle, or SBR.

IMHO, your two best paths are:
- Buy an SBR (or SBR lower) from a dealer. Pay $200 tax, wait forever for the transfer to be approved.
- Buy or Build an AR pistol with the length barrel you want. Shoot it a bit to make sure it works right, then file an ATF Form-1 to make it an SBR. Once the F1 is approved, put a stock on it and have it engraved.

BUT - didn't your question focus more on preferred lengths  for the barrel?

IMHO, AR barrels break down into three groups.
- Those that are or can easily hit a minimum 16" length. No extra paperwork needed, as they're normal rifles.
- Those that are super short. These are more novelty and/or pistol calibers. To me, this is anything under 10".
- The middle ground. 11.5", plus or minus an inch or two. I'm partial to 11.5" and 12.5" for both practical and aesthetic reasons.

Just FYI, this is not possible.

A lower alone is not a SBR and therefore can't be transfered as one.

My very first SBR was a stripped lower that a local 07/02 sold me, by the time my form 4 was approved they had got a talking to from their local IOI during an inspection that they couldn't sell a stripped lower as a SBR for this very reason.

When I went to pick it up, they gave me the option to pay for the parts they used to complete it or I could disassemble it and keep the stripped lower and lower parts kit worth of parts for the original price I had paid and they would buy back the rest to cover the difference.

Unless the dealer ordered it on your behalf or is making it, registering it and transferring it to you on a Form 4 , there is no reason to involve them.

For a Form 1. (You build/create)  I (and most here I think) would
Get a Lower you really like and plan to have for the long haul, plan out what upper you want (for the length part of the form)
Register the lower on a Form 1,
Once approved, have the appropriate engraving done,
get your upper, slap it on the lower, put pins in,
enjoy.
Some will argue about if you have to engrave it or not.
Some will say you can't have an unregistered lower and a pistol length upper, even not assembled as it shows ""Intent""...
If you are that concerned about it, order a Pistol Tube (not Pistol brace) and put it on the Lower, till your stamp comes back approved.

JMHO...  
With this crap there are  some grey areas and maybe more "I don't like or recognize the law".
Just know what you are doing and decide if there is a risk.