AR upper question
Hello all. I have a question on a AR upper? I know this has been asked before many times but from my research it's not answered my question. So, does the Fk marking on the upper stand for FN or bushmaster? Is the case that FN made it and sold it to bushmaster or is it really a FN upper? I've looked up the codes and some say FK=bushmaster and others say FK=FNMI? So how do you tell the difference? Also, this is a Frankenstein gun were talking about barrel upper and lower aren't from the same place so it's not a bushmaster AR. Any help would be greatly appreciated.