I got very acceptable results from an even easier process than Kutter's.
Polish the trigger guard to 400 grit or better,degrease, coat with linseed oil, dust with hard wood ash (fine ash like talcum powder) and hang them in your gas bbq grill at 600 degrees. Remove and dust off the part, soak in oil for 24 hrs. The finish is durable and accurate in coloration for a restoration.
Easy and repeatable.