Pete,
Acetone will not bring out the contrast. I'm certain the intent is to clean and degrease.
Concerning your barrels. Often, near the end of the process, you can gain a little more contrast by using a mild abrasive such as rottenstone or fine pumice. Oscar Gaddy used Bon Ami cleanser. I've found that it's in the abrasive, not the cleaning agents. You could try this, below the forend perhaps, to see if it will help.
Jim
You'll need to remove any protective finish first, of course, and this in itself may freshen things up a bit, if the protectant has darkened with age.
