Shaft collars as bushings?? Lathe boys i need some advice.

I have been looking at alternative bushings, one that a person can make themselves. I have found shaft collars to be what i think they at AFuse for bushings. Only thing is they come in a 7/16″d opening and 9/16″ opening. Obviously i’ll need to open up the 9/16″ so i can use it on my 18″ barrel. Is a 5/8″,.625 drill bit close enough for the 16mm barrel thats 15.875mm. Also i do have a drill press but am worried a bit about being perfectly centered and concentric to the bore of the barrel. Should i be so worried, as i’m only removing about 1/16″ of material or 1.587mm. I don’t have a really nice micro, i know i need one, but any info would be much appreciated. What does tha dma 18″ barrel really measure and how close do i need to be. I’d like to be right on if possible. .6299″ is 16mm what bit would i want to purchase for this operation??