okay, i am a newbie here on the site and a newbie in building AR's. I am looking for a 16" barrel on Gun broker and I am not sure what M4 feed ramps are referring to. I have the upper and lower ready, just need the barrel. Any ideas what a seller means when they say "Does NOT have M-4 feed ramps". if i want a 16" barrel what should i be looking for? thanks for the help!!!