I have an upper with a small front pivot pin hole that I want to attach to a lower with a large pivot hole.  I know I need the adapter pin, but does this affect the "play" between the two receivers?  Is there a drawback in attaching these two receivers?  Thanks!

i just recently had to do the same thing myself. i found no play in the upper and lower when mated together, but it also has not ben shot yet. so cant comment on that.

I have been using a small hole upper on my FA lower receiver with a large hole. No problems at all.
There are two types of conversion pins available. I have the one from Bushmaster, which from what I have read here is the least desirable. Everytime you open it up you need a screwdriver to reposition the pin. I don't know anything about the other type but I am sure someone will follow up with it. From what I understand it does not move so not repositioning necessary.