Sorry - I had another brain murmur as I was typing the following. I should have read your post as it was written and not as my obviously dyslexic mind saw it. I thought you were cleaning up and old side plate, not a new casting. The following doesn't apply AT ALL to your question, but typing is so laborious for me to do with two fingers, I will leave the process for anyone interested in cleaning up OLD parts.
I have had very good luck with the following very mild process:
1) Degrease the part with acetone or an equivalent solvent and a soft brush (tooth brush is good as long as it made of something that is not dissolved by the acetone.)
2) Wash the part with dish washing detergent, hot water, and a soft brush.
3) Put the part in a dish of vinegar over night. The acetic acid in the vinegar will convert most of the verdigris / oxidation to a form that will come off more easily.
4) After the vinegar treatment, rinse, and dry. If you want the part to look fairly new, brush it lightly with a soft brass brush (Brownells has these as do most machine tool supply houses and some hardware stores). If you want it to look clean, handled, and well cared for, but not bright down in the crevices, just buff it with a piece of denim. This will shine the high spots without removing the converted (usually reddish) oxide layer down in any of the low spots. If the denim alone does not brighten the high spots enough, use Dave B's trick. Wet the denim and put on a little tooth paste or baking soda and scrub the part with that. (A more aggressive version of this last step is to use, in ascending order of aggressiveness, rotten stone, Comet cleanser, or pumice.)
Different brasses with different types of verdigris / oxidation react differently to all of this, although I have never seen this process really mess anything up. However, if there are any hidden or smaller brass parts on the gun that you can try this on first, that would be a good thing to do.
Dave C
