First off im new here and this is my first post. Id like to start off by saying i have build twenty or so lowers five of them being my own. I have two completed one is a 22lr and the other is your standard 223 556 that iv tricked out. This is my question, i have always bought upper receivers completed but now im wanting to build my own since it is a money saver and i enjoy doing things on my own. The build i want to do now is for my ghost gun lower in od green. i want to do a 13.7 inch barrel with the kx3 flash suppressor for a total of 16 inchs of barrel. (it would also be tack welded) now before i go into detail with parts i want im wondering if you guys could help me with the gas system a lot of people tell me it can be tricky and blow up in your face if its done wrong. So tell me how hard it is to do an upper. and which lenght gas system woud work for my future build? it is going to be 223 556 cal just incase you were wondering. thanks guys let me know and ill post some pics of my guns iv got in a new threed.

It's not really the hard to build an upper.  You'll probably want to go with a carbine length gas system and get gauges to check you barrel's headspace.  Other than that and the appropriate tools/fixtures, you should be gtg...