I think it will have a more negative impact than firelapping the bore to clear out the rust would. If the rust is all at the breech it may be due to having been improperly cleaned after firing corrosive primers long ago. In general, for lead bullets, a slight taper from breech to muzzle of a half thousandth or thereabouts can be good. Only the reverse is bad, so unless the rust is really deep, this may be your best solution.
If it were my gun, I would operate as follows: Paint the crown and end of the barrel with nail polish to protect whatever is left of the finish from drips of rust remover. Plug the muzzle tightly with a Neoprene stopper. Clamp the gun muzzle-down. Use an eyedropper to fill the barrel with Evaporust or Rust Release without letting any get on the outside finish. Do not get any on any bluing. This will need about 15 minutes to remove the rust. As the end of the time draws near, set some distilled water to boiling in a kettle. When it reaches a boil, hold the gun over a drain and pull the cork, followed immediately by flooding the barrel from the breech end with the hot water and pouring it over the muzzle, too. That will remove the rust remover and dry off fast without rusting anything. Then hose the gun with WD-40 to displace any water trace, then wipe that off and apply gun oil (WD-40 will make the gun sticky if you don't wipe it off the outside, but you can leave it in the bore.
You'll find the metal a little rough where the rust was. Firing polishing loads will fix that. If you reload, take some cast bullets and use a toothpick to remove the lubricant in the lube grooves and put some JB Bore Compound in its place. Load a bunch up with about 1 grain or 2 of Bullseye or make a similar cat sneeze load. Fire a couple dozen of them to polish the bore up some.
If you don't reload, Flitz on a patch on a cleaning rod can do some of that for you, too.
