If someone in the "know" told you that, you need to find another someone who knows to listen to.
Yes, you clean the copper out routinely IF you want any degree of accuracy. Now if that someone is a gunsmith my guess he is just waiting to sell you a new barrel.
Wipeout Accelerator Patch Out, Montana Extreme(ME) 50 BMG are both excellant and both take the copper and the carbon ring in front of the throat. It is essential to remove both and not all products remove both.
Good one piece rod, I like Deweys coated rods.
Bore guide to protect your chamber leade and bore. The best is the Lucas for the money.
ME plastic bristle brush
Jag correct size, I like the dewey pierce that you can pierce or wrap.
Patches to fit with the jag and bore.
Follow the directions and follow up with a patch of ME Bore Conditioner or Kroil, then two dry patches. I like the ME best. My first shot is always closer to my zero than with kroil.
All can be found at Sinclair International - Specializing in products designed for precision rifle ammunition reloading and shooting
or Robert W. Hart and Son Inc.