The technique is called dry-brushing. You can use other colors than silver as well.
I just find "i want even more realism" a bit vague.
If you look at a "standard" black gun there's wear (which you have done by dry-brushing), there's rust, scratches, chipped paint and different stains, like powder residue, gun oil and such.
Very much depends on what the gun is supposed to be made of. Nickel-plated gun ages completely differently.
So, what does "more realism" mean to you?