There are commercially made hand held fixtures to attach a bbl or a magazine tube to to 'spin polish' the part against a slack belt Belt Grinder.
They are simple. Usually aluminum to keep the weight down.
A bar with a standing center at one end and an adj for length center on the bar.
Many 'smiths make their own and use nylon(?) centers to avoid wear on the bbl while spinning.
Place the part betw the centers which are usually roller bearing on the commercial made fixtures so they spin easily. Push the part against an abrasive belt while it's running and the part spins against the belt and gets a nice clean polish.
They can be tricky at first. You have to slow the spin RPM of the part with your thumb on one end. Otherwise the part picks up rpm speeds far exceeding what you want. Scary high rpm. I've seen a bbl or two fly off of one of the fixtures while being used. Spinning wildly fast and either the centers give out or the adj end center is not tight enough.
Lots of noise and a lot of fix up file work on a bbl that gets loose and runs around the shop floor banging into things.
On the other hand,, you don't want to stall the spin while polishing as it will leave a flat spot in the bbl as the bbl stops moving and the abrasive belt keeps cutting.
Trying to spin polish that flat spot away can lead to the same problem sometimes as spinning the cylinder and polishing over the flutes. It just makes it worse.
You have to be careful running over dovetail cuts so that the belt doesn't cut one or both of the dovetail edges away as you go over it. Kind of the same problem as you can get with simply spinning a cylinder and leaning a polishing stick against the the end with the flutes.
It cuts the edges away real quick, but it polishes quick too!
Lots of factory bbls mag tubes and such shaped parts were still polished in this manner(spin polished) but they don't use a fixture to hold the part.
The polisher usually sat in front of the polishing wheel, the wheel could be as large as 3' in dia.
On one knee the polisher had a simple wooden block attached with a strap. The block would look like it's been used for years and probably had.
A slight 1/2 rd depression in the block allowed the polisher to place the bbl and then with his knee press the bbl against the polishing wheel. The bbl spun and polished as the polisher guided the bbl from one end to the other across the wheel.
The polishing machines were set up with a wheel on both sides.
The polisher on one side would be polishing with a coarse grit.
When each bbl was done he simply placed them in a small rack betw himself and the other polisher on the other wheel.
That other polisher would then polish the same bbl again using the same technique, but the wheel he was working on was charged with a finer grit.
They would do that all day long....
Other polishers were set up at various stations using hard backed wheels and some belt grinder setups for polishing other parts. Each station set up to do one grit on usually one specific part or surface shape.
I'd guess most of the operation is automated now. Vibratory polishing and simple bead blast finishes are acceptable to buyers. So the labor intensive hand held machine polish is probably nearly gone.
Tough to find someone who would want to do that in a factory setting these days as well.