Pete,
The horizontal/vertical and disk sander machines are good all around shop tools and they work ok for sharpening, recoil pad installations, etc., but for barrel polishing and knife sharpeningI have found that a little "slack" in the belt is where barrels polish best. This won't work on the wide belt machines unless you can find a way to remove the platen from behind the belt. I use a 2x72 Wilton square wheel grinder and polish rounded surfaces in the area between the idler pulley and the wheel. This is a rather expensive machine, but I made knives for a number of years and it held up pretty well. Any belt sander with a fairly long unsupported section of belt that you can get the work properly positioned to should work better than any hard surface platen type. Some guys build little belt sanders that mount on the lathe carriage - you can run the slack part of the belt up and down the barrel using the lathe feed (need to protect the lathe ways big time...).
Scott