I'll try and make this simple since I wrote that abomination about hammers in the FAQ. it is only from what I understood from others.
Your bushmaster is most likely equipped with a "notched" AR-15 hammer. That is to say there is a small nothch cut out of the top front of the hammer. If you mount a Colt style 9mm upper (such as a Colt or Rorck River 9mm upper) and do not change the hammer what is going to happen is you will most likely get one shot off and the hammer will bind up against the bolt and you will need a hammer and punch to get the receivers ap[art to fix this mess.
There are two solutions to perevent this.
1. Hammer replacement
A. Replace the hammer with a "DPMS" style AR-15 hammer. It looks exactly the same as the hammer in your Bushmaster EXCEPT that there is no little notch in the upper front face. Your gun should run fine with both 9mm and .223 uppers. The hammer in the pic above is similar in appearance to the DPMS AR-15 hammer.
B. Replace the hammer with a Colt or Rock River 9mm hammer. Will work fine with 9mm upper, I have heard rumors the hammer may not reset properly with a .223 upper
C. Get an M16 hammer and cut off the "tail" that sticks up, such as in the picture above. Should work fine with both 9mm and .223 upper. As for the legality of getting an M16 hammer in Finland I can't say.
2. Bolt ramping. When the botl is ramped the weapon should work OK with all hammers EXCEPT the 9mm hammer- the 9mm hammer won't reset sometimes.
OK, hope that helps. If anyone see a mistake in the above please let me know.
Dawg