I ordered all of the parts to do my first build. The parts kit comes with the FA trigger. What do I need to do to keep the gun semi auto? Do I need to order a semi auto trigger or is there something about the reciever that makes it semi auto?
If it's a rifle, you prob will want to change it out to a US made semi one to keep compliance with the 922 rules.
To answer your question, the full auto group that comes with the gun will inlcludes a hammer with an extra notch to engage the sear, the sear, and a disconnector that has an extra feature on the back to engage the selector. You obviously (and legally) can't mount the full auto sear in the new reciever, so the fire control will not funciton in full auto. You will want to grind off the rear section of the disconnect to so there is no way the selector can interfere with it.
Remember, the full auto fire control also worked as semi in the orig gun.
But as stated above, it's a moot point, because to comply with 922, you need US parts, and the fire control group is a cheap way to get 3 US parts...
...be safe, be legal, read the tutorials.