So I had an upper built using parts I had. A BCM upper receiver with a Colt carbine length skinny barrel [16"]. It runs great but I had to run the DD A1a rear sight all the way to the left to get it on target [36 yard zero] . I don't doubt my gunsmith who put it together or the parts, all quality. So my question is: what can be done to get a zero more centered in the sight body? I have a Magpul plastic rear and a Colt A2 carry handle rear to try out when I get the chance.