I've noticed also that some of the trigger's call out for specific pin sizes but I don't see the sizes listed on the stripped lowers. I'm guessing I would have to call for that info on some of them.
I'm sure this is a dumb question, but what is an FFL. I've never ordered a firearm online. I do know that you have to have it shipped to a local licensed gun dealer. And is it correct that the only part of an AR that needs to be registered is the stripped lower?
It would be dumber to not ask.
An FFL is a Federeal Firearms License [holder], what you are referring to as a licensed gun dealer. We are required to use them for most transfers across state lines. Since the lower carries the serial number, it is the firearm; everything else is just something you hang on it and can be ordered from and delivered directly by the vendor.
My sense about the pin question is that so long as one stays away from Armalite, all the small pin receivers use the same pins. Someone with a deeper sense of history on this can explain why a government contractor might switch to a non-standard pin only for civilian sales.