I certainly have not chambered as many barrels as many people here, but it is my humble opinion that when you get a barrel blank, no matter whom the supplier, you cannot assume that the cylinder portion of the barrel will be concentric with the bore. Chances are it'll be close but not as close as you might like. I run my steady rest on the tenon, even after my threads have been cut, and I've not had any trouble from wear of the steady rest fingers during a chambering job. I do pay close attention though. I don't set them tight, I initially set up with .002 shim stock between the fingers and the tenon, then carefully remove the shims and take out the final clearance by feel.
I think if you didn't want to run on the threaded tenon you should do a clean-up cut on the cylinder portion while the barrel is set up in your centers before you set it in the steady rest.
If you don't want to do that, or otherwise mess with the barrel's profile or finish...I've done the following but it's not my idea. I read it years ago and want to attribute it to Ed Shilen, and I want to say it's in the “130 page PDF of different chambering methods” linked to in the Advanced Gunsmithing and Engineering forum. But I'm not certain about either of those. Anyway, an aluminum cylinder is bored with a hole to fit around the barrel with some clearance. It is affixed to the barrel with polyester auto body filler. Like Bondo. With the barrel between centers, the OD of the cylinder is cut and will be concentric with the centers. The steady then runs on the aluminum cylinder. When the job is done, heat from a heat gun takes the Bondo off. I've done this on blued, finished barrels and not left a mark.
I think if you didn't want to run on the threaded tenon you should do a clean-up cut on the cylinder portion while the barrel is set up in your centers before you set it in the steady rest.
If you don't want to do that, or otherwise mess with the barrel's profile or finish...I've done the following but it's not my idea. I read it years ago and want to attribute it to Ed Shilen, and I want to say it's in the “130 page PDF of different chambering methods” linked to in the Advanced Gunsmithing and Engineering forum. But I'm not certain about either of those. Anyway, an aluminum cylinder is bored with a hole to fit around the barrel with some clearance. It is affixed to the barrel with polyester auto body filler. Like Bondo. With the barrel between centers, the OD of the cylinder is cut and will be concentric with the centers. The steady then runs on the aluminum cylinder. When the job is done, heat from a heat gun takes the Bondo off. I've done this on blued, finished barrels and not left a mark.