Going to start a new Varmint build.  I varmint hunt quite a bit in the spring, and in a good day will shoot well over 1k rounds. It's usually a mix of some 22LR and 223.  I typically take several rifles to allow for cooling.  My go to 223 is a Cooper Montana Varmint.  I have a 20" bull barrel AR that I set up mostly for coyote hunting that I use occasionally, but I want to build a dedicated AR squirrel gun.  With the Cooper I shoot squirrels out to 400yds on occasion, and 200-300 is pretty common, so accuracy is important.  

There are days I like to use my Gemtech One suppressor, so a barrel that is already threaded would be great, though I can get it threaded locally if needed.  The barrels  at the top of my list right now are a Lothar Walther Var Tac and a Rock River varmint.  The Lothar Walther barrel is threaded as standard. RRA has a 24" barrel that comes with standard threading, but their 18&20" don't.  They are both 8" twist, which is fine as I shoot mostly 55gr bullets.  Since the gun isn't toted around weight isn't an issue.  I really like the 24" RRA barrel, but by the time you add a 7.5" can on the end it would end up pretty long. I also like that all of the RRA barrels are cryo treated.

Any suggestions or advice would be greatly appreciated.  The upper and lower will be Seekins Precision.

Of the two get the Lother Walther barrel, they make a good barrel. Walthers are the barrel the LaRue used for years to build their reputation for accurate rifles. That was why LaRue had a lot of work and a big learning curve when they decided to make their own barrels.

I have a couple Rock River rifles, none have a Rock River barrel on it. My High Power rifle built out of a Rock River DCM rifle has. Kreiger barrel on it. For an iron sight rifle it is real accurate.

RRA varmint barrels are made by Wilson Arms. You can check out numerous Wilson barrels at...

http://www.jsesurplus.com/

No experience with the rock barrels.I do own a Odin, a lothar Walther, and alot of other factory barrels.To tell the truth, while alot of my barrels are nice, and accurate,None hold a candle to my JP.If accuracy is your main goal in a AR,I wouldn't even think about other options.

My 20 1/2'' Wilson barrel (non threaded) and originally 24'' is as accurate as a barrel can get. It is for sure sub 1/2'' MOA capable.
I have been completely amazed how accurate and consistent it is.  Other aspects of the AR it is on are surely part of the accuracy but the barrel is certainly not anything that is lacking.

I hadn't considered a JP barrel, mostly because their medium contour barrel is rather thin.  They are cryo treated and do say resistant to thermal drift, but I question how well they would hold up to some continual shooting.  Might be worth considering.  

The Lothar Walther has probably been at the top of my list.  I have a couple of their barrels on bolt guns and have been happy with them.  I have also considered a White Oak barrel.

White Oak Armament (WOA) has several barrels that would fit your needs