As a general rule it's best to do all barrel work prior to cutting tenon and chambering. Anything that can add stress to the barrel and even remotely affect the alignment of chamber to bore.
Blank looks like someone tried to thread the entire barrel
Ok, were going to attempt to ejumacate the masses here. We all have our quirks. Mine is a personal crusade against spreading disinformation regarding "stress" in steel.
MACHINING DOES NOT INDUCE STRESS. Don't care who says it does, they are wrong. You RELIEVE stress when you machine. When you use a tool that removes a piece of material you are relieving stress. Period.
The analogy I use to explain this:
Make a ball of rubber bands. Snip random pieces with scissors afterwards. Do enough and ball is no longer round. Did you induce stress? No, reduced it. You just had no assurance the ball stayed round.
Now take the same ball and cook it in momma's oven at 200F for a couple hours. Now snip at it. Nothing happens OR nothing alarming happens.
Why?
The parent material has been NORMALIZED. aKa Stress Relieved.
Such is the same with barrel steel. It's normalized so that flutes, contours, etc don't turn the stuff into a banana.
Now, if you take that same ball of rubber bands, warm it up, and then pound (forge) the piss out of it till its in the shape of a box, then who knows whats going to happen when you cut on it. Why? You altered the parent material into a shape it's not normally going to form. So, STRESS has been created in the part. When "smoosh smoosh" a piece of material into a shape you ARE creating stress into that part. Forged cranks, wrenches, etc experience this. It's why they are normalized post forging and THEN machined/ground, etc. It's so that you have a reasonable expectation of the thing retaining its dimensions afterwards.
If you normalized it afterwards by heating it back up and allowing it to relax, then again, when you machine on it, it should stay reasonably close to its parent form.
To the OP:
Spin/contour your barrel and be fearless of ruining your barrel. You'll be fine.
A spinner is nothing more than a pair of bearing cassettes fitted with plastic (sometimes brass) dead centers. It's mounted on a 1x1 steel box tube rod. You rub it up against a belt sander and polish the inclusions out of the barrel surface. It takes less than 5 minutes to bring a barrel up to a nice even finish.
Advantages:
1. NO abrasive media anywhere near your lathe.
2. Very fast
3. Superior finish (with practice)
4. For a shop with multiple people using one piece of equipment it moves an operation somewhere else so the machine stays productive
5. Safer. No worries about fingers getting in the chuck or a belt wrapping itself around the barrel and pulling your hand in.
Disadvantages:
1. Takes some practice to get the hang of it.
2. #1
3. If you DON'T know what your doing you can give yourself a really nice broken nose AND ruin the barrel.
4. Requires purchasing an additional piece of equipment you may not already have. (belt sander)