If your heart is set on ordering from Evike and you don't have a ton of money to spend I would recommend a Systema tightbore. It's not as tight as other barrels but they are well made and you will notice an increase in accuracy. They have M16-length barrel's for $45:
http://www.airsoftpost.com/systema-tight-bore-high-performance-straight-inner-barrel-500mm-m16a1a2aug-p-24410.html
If you want to spend a little more then Prometheus and KM are the way to go. Here's the Prometheus 509mm:
http://www.airsoftpost.com/prometheus-509mm-603mm-barrel-m16a1a2-p-24390.html
To answer your question about Teflon coating: yes. The Teflon makes for a very smooth surface. I have a KM Teflon-coated barrel in my M24 and it performs very well.
http://www.airsoftpost.com/systema-tight-bore-high-performance-straight-inner-barrel-500mm-m16a1a2aug-p-24410.html
If you want to spend a little more then Prometheus and KM are the way to go. Here's the Prometheus 509mm:
http://www.airsoftpost.com/prometheus-509mm-603mm-barrel-m16a1a2-p-24390.html
To answer your question about Teflon coating: yes. The Teflon makes for a very smooth surface. I have a KM Teflon-coated barrel in my M24 and it performs very well.