I view one armed chins as if they have legendary status, they are very rare and I have never known anyone who could do one. I set a goal for a one armed chin a long time ago and to train I did weighted chins and I did this for a long time. I got up to 50 pounds at one point but lost interest as I pursued other things.
Today I discovered that I am closer than ever to performing one. I tried doing weight assisted one arm chins by slinging a rope around a smooth bar so that it acted as a pulley and then I attached 20 pounds to one end. I was able to do a chin without too much difficulty. The last time I tried this I could barely get up with 40 or 50 pounds.
This leads me to believe that I can start doing more specific one arm chin-up work using the method I just described and one arm chins with an assisted rope pull by the other arm.
I will achieve this bad ass skill. It is only a matter of time.