Same stuff you use on your barrel. You can rub back browning or bluing for a nice grey. Let browning go a bit wild and you'll have a slightly pitted grey. Use something like LMF browning for a light aging to a chocolate brown, smooth or pitted, rubbed back to where you like, if you like. Bleach bath. Etc. Folks will pop in with a million ways to use a million different products. I will say knock back your edges if you have an oct/rnd barrel as nothing looks worse than age and patina on a sharp edged barrel. JMHO.
