I prefer that people pull the barrel off and send it to me, but I often take apart a whole upper if the customer does not have the tools to do it.
I usually remove the front sight base as well, just makes it easier to hold the barrel through the headstock on the lathe when I re-cut the crown.
If you are considering swapping the front sight base out for a flip-up setup, then it's coming off anyways and that makes it an excellent time to do the swap.
There are a few options out there, ranging from a simple YHM or PRI flip-up front sight base that replaces the standard FSB on the barrel, to a gas block with a picatinny rail that you can add either a flip-up sight or a removable sight tower to.
Lastly, you could go to a quad railed handguard system with a flip-up sight on the rail.

PRI is probably the best flip-up front sight that's barrel mounted, but YHM is half the price and very serviceable.
I think a dedicated barrel mount flip-up is the best option, much better than a railed gas block AND some kind of sight added to the rail.

Some pictures:

YHM flip-up:


PRI flip-up:


YHM gas block mounted flip-up:


YHM gas block mounted tower:


YHM handguard mounted flip-up:


There are LOTS of other options, these pictures were just easy for me to find to show you ideas.

Quoted: also some require removal of bbl. but adcofirearms does not, think there are some that can actually chuck up a whole upper and thread.

I'm still trying to figure out how they do that.
I have an idea based around a LONG keyed center that will reach down inside the reciever to the barrel extension and drive it from the lugs.
My issue is with the crown.
I poke my live center in the old crown and this damages it.
This is the only way to ensure that you are threading perfectly concentric to the bore.
I suppose adco has a brass tipped live center and this could work as well.
My method with a steel live center requires re-cutting the crown when I am done.
This has improved accuracy in some cases as a side benefit...

Quoted: I have an adapter that allows me to chuck up the assembled upper. But, I really prefer to work on a stripped barrel. Less problems and less that Mr Murphy can screw up. Charles the Gunsmith.

Charles-

Am I half way there on my arbor idea down the middle of the reciever and engaging the barrel extension?
Got any pictures to share?

Quoted: You really have the right idea as the adapter is a steel cylinder the exact OD of a bolt carrier. There are 2 drilled and tapped holes in the cylinder that allow a clamp with 2 matching non-threaded holes to have hex head allen type bolts inserted and the clamp bears down on the bottom of the upper receiver. EDIT TO ADD: No real need to engage the barrel extension with this cylinder and clamp.

So it's just a 0.995" diameter bar with two 1/4-20 holes tapped across it and then a couple little plates clamp the reciever against the bar?
That's EASY.
I was trying to be all accurate by centering the barrel on a point at the end of the bar and basically holding the barrel between centers and letting the reciever float.
As that setup clamps the receiver to the side, I could certainly run the steady rest for crowning.
I already have a sleeve I thread on to the muzzle for crowning when I can't get the FSB off, so I am half way there.
Thanks much for the pointer...

Quoted: Quoted: also some require removal of bbl. but adcofirearms does not, think there are some that can actually chuck up a whole upper and thread. I'm still trying to figure out how they do that. I have an idea based around a LONG keyed center that will reach down inside the reciever to the barrel extension and drive it from the lugs. My issue is with the crown. I poke my live center in the old crown and this damages it. This is the only way to ensure that you are threading perfectly concentric to the bore. I suppose adco has a brass tipped live center and this could work as well. My method with a steel live center requires re-cutting the crown when I am done. This has improved accuracy in some cases as a side benefit...

The guys who do all my shop work were threading barrels for 2 years before I ever saw how they were doing it.  Its pretty neat, really clever, and has one of them fancy machine shop things dedicated to cutting and threading AR barrels.  We lost track over a year ago over 2000 barrels threaded, so it must work.
I wouldnt even know how to turn the thing on

Quoted: I'm not a machinist but I am a woodworker and have a basic understanding of metalworking.   So, why can't you fix the barrel in a chuck/jig and spin the cutter/die around the barrel? Just curious.

You can use a die which is soecific to a certain thread spec, but a die does not cut threads nearly as nice as a single point tool that makes many small cuts.

Here is a barrel blank after threading for the barrel extension.
You can see that the tool cutting the threads is a single point cutter.
In use, you plunge the tool into the metal and the lathe moves it across at the proper rate to cut the threads.
You make many passes, cutting as little as 0.001" on the last passes.

Quoted: Nice looking threads Randy. Charles.

I like this picture of muzzle threading better

Quoted: Thanks for the lesson guys.  Very informative...and the pics are great! One more question:  In order to make multiple passes at thread cutting, both the material and the tip of the cutter would have to be returned to exactly the same starting point each time.  Is this a feature that is built into the lathe??

Yes.
You have a thread dial that tells you when to engage the threading drive.
As the dial comes around to whatever mark you are working from, you engage the clutch.
The drive will take the cutter across in exactly the same place each time.