Hi, looks like I might be able to help. Are you just looking to dirty it up a bit, or are you looking to beat it up?
The first thing I'll suggest is testing these ideas out on something you don't care about too much.
If you just want to add dirt, think about where the dirt would tend to gather. Make a wash (some black paint that's been really thinned out) and apply it to the piece. Before it has a chance to dry, wipe it down with a cloth, but not too well. This should leave some filth in the corners and gaps. You may also notice that dirt sometimes gathers immediately beside heavily used areas. You can use the wash technique there as well (grip, trigger) but use your hand as the tool/template.
If you create any dings or scratches, the same wash will work to darken those as well.
As for powder deposits/burns, it would be better for someone else to answer. I've got ideas, but they might be...destructive. I've done similar things that have involved real black powder to create an effect, but I don't think it wise to dispense that sort of advice.