I want to make a 16'', relatively light weight (I think) rifle that is fun to move and shoot.
16inches and light weight means a light weight barrel (or at least a light weight profile under the handguards.
What I need help with, is what parts I need, what I'm missing, and where to get said parts I don't have.
I would look at del-ton.com if you want a reasonable cost reasonable functional parts build.
I just bought a lower (with lpk installed) and an assembled YHM A3 upper reciever.
This doesn't make sense. I see your picture below. It doesn't match the above statement. I am confused.
What do I need to look at in regards to midlength vs. carbine length vs. rifle length gas ports?
Rifle length are for 18-20 inch barrels.
For a 16 inch barrel semi-auto you can do carbine or midlength.
I need a BCG and charging handle of some sort.
A standard charge handle will do. Since you have one, try it out in the new rifle. The gas buster is very nice. I bought it because I was running a suppressor.
Do I need the rail to be 12'' or so, or can the sight be more toward the rear? Does this matter for anything other than sight radius? Would I be looking at either of these?
No you do not need rifle length rails, but if you use a low-pro gas port is looks really cool. Yes, I have a build like that. It is heavier. It does allow for more stuff on the rail and push the front sight out there.
Do these come with the pins or screws needed for install?
Yes, they come with everything necessary for installation, but some barrels are setup for cross pins, some are setup for screws.
The low pro will fit under your rails.
Do I need a muzzle break or comp?
NO, you do not NEED a break or comp to make the gun function. Breaks suck in general. I like vortex flash suppressors, but I really have not done a test at night.
Am I missing any other parts?
Yes, you need tool and how do you plan on attaching a sling/single point.
To be perfectly honest, it is cheaper to buy a complete upper now days unless you want something specific.
One day, I shall come back. Yes, I shall come back. Until then, there must be no regrets, no tears, no anxieties. Just go forward in all your beliefs and prove to me that I am not mistaken in mine.