One more thing, the super cheap copper tubing sears is selling as "utility coil" is not ASTM rated. I contacted the manufacturer and they told me it is .020" wall thickness. For reference, type M tubing is .028" thick, type L .040", and type k .049. These numbers are for 1/2" tube and would be different for 3/8" tube.
I don't know what the od would be for the sears tube. It could be 1/8" larger as is standard for l, m, k tube or it could be as stated (i.e. od=1/2") as it is for ACR line. Or it could be something else entirely. I dunno. If I was on a budget I'd consider the sears "utility coil" but I think I'll just fork out the extra cash for something a bit better.
I bet type M tube would be fine but I'm going to try and locate some ACR line or type L tube. The sears stuff is very thin walled, it might kink easy or not. I dunno. For $26 shipped for 1/2" X 25ft it is a heck of a deal and probably worth trying.
Oh, and for the record I'm getting this info from a handy .pdf manual located at
http://www.copper.org/resources/pub_list/pdf/copper_tube_handbook.pdf
You can look up the 3/8" stuff there if you like.
Edit: sorry for all the 1/2" references in a thread about 3/8" copper tube. I'm shopping for 1/2" right now so that was what was on my mind. I know this is off topic but I wanted to put it out there anyhow and it certainly doesn't merit a new thread. Hopefully it's helpful for someone. And the cheap sears utility tubing is available in 3/8" as well.