SETUP:

Bushmaster lower - 9mm RRA hammer installed by myself, and RRA 9mm mag block. Fired about 200 rounds and got home and saw that the pin had worked itself loose.

Look at your manual for the diagram of the trigger assembly. You probably don't have the legs of the trigger spring on top of the trigger pin. (Common mistake) There are notches on te trigger pin for this purpose. (Spring engagement in the grooves of the trigger pin).

If you look at your trigger group, I'd wager your "Spring Legs" are under the trigger pin.

Many use the Knights Armament "Anti Walk Pin Set" in addition to a properly installed trigger group. Many say it's not necessary, but some (includuing me) have had their hammer and trigger pins walk and this is a sure fire way to keep it from happening.

Tack

www.ar15.com/forums/topic.html?b=7&f=25&t=184095

Quoted: Look at your manual for the diagram of the trigger assembly. You probably don't have the legs of the trigger spring on top of the trigger pin. (Common mistake) There are notches on te trigger pin for this purpose. (Spring engagement in the grooves of the trigger pin). If you look at your trigger group, I'd wager your "Spring Legs" are under the trigger pin. Many use the Knights Armament "Anti Walk Pin Set" in addition to a properly installed trigger group. Many say it's not necessary, but some (includuing me) have had their hammer and trigger pins walk and this is a sure fire way to keep it from happening. Tack

It looks like the spring legs are sitting on top of the trigger pin - however it still seems really loose. I can just push the trigger pin on the outside of the rec'r and it wants to move.

+1 on the KNS pins.

One more thing, make absolutely positively sure you're trigger spring is installed correctly.

Quoted: +1 on the KNS pins. One more thing, make absolutely positively sure you're trigger spring is installed correctly.

I'm pretty sure it is. I never took it out - it was factory installed.

I guess you could call up Bushy and see what they say. If they can't give you a good solution (new reciever, new pins, and/or new springs) then KNS pins are probably the way to go.

Hammer pin walking out?

Lack of J spring tension in the hammer or too small of an annular groove on the pin.  

Since this is a 9mm, you don't want to deepen the annular grove in the pin since it will just cause it to snap faster.

If you can pull the J spring in the hammer, then you can retention it, remembering to re-stake it back in the hammer when you reinstall it (helps to taper point the leg of the spring to make sure it locks into the pin groove.

Or as other have pointed out, the KNS set.

Myself, since I have broken, and have seen way too many trigger/hammer pins broken on the 9mm rigs (read non ramped bolt), installing the KNS pins in the 9mm have become standard practice.
Note: unlike the c clips pins, the KNS pins do not rotate, and you don't end up with circle scraps on the receiver sides that the C clips can produce.

Anti walk pins will fix that. If you don't want to go all out on the KNS, you can go cheaper with a set of DPMS anti walk pins for about 1/3 of the cost.

This is not a 9mm set up, but the pic. might help.