I find that lots of ramrod grooves need to be lowered (towards the barrel) on pre-carves. That is pretty easy to do to the exposed groove, but the section that is hidden is obviously tougher.
I've opened them up a little by getting a rat tailed, round rasp of the appropriate diameter. Heat up the tail and bend it up to 90 degrees. I glue a 2 inch section of 3/4 inch dowel onto the bent tail to make it more comfortable to hold. Put the barrel in, flip the gun upside down, and press down towards the barrel while you work.
I don't know if this will work to take off as much as you are talking about though.