I am looking to build a hunting AR15 that is accurate out to a few hundred yards and light enough to carry around comfortably; something I can not do well with my AR10. I'm pretty much decided on 6.8, as I have no plans of running a can so no need for the 300BO/whisper.

So I started ordering some parts: Midwest Industries billet stripped upper and PTac 6.8 bolt from PSA. I'll probably go with the MI-SS12G2 or the 15" version.

Now I am considering barrels. I am looking on ar15performance, but if there are other barrels I should consider please chime in. I'm considering going with a 16", but are there any advantages at all in an 18"? My last barrel question is to get threaded or target crown.

If u can find one , WOA (white oak armament) makes some fine 6.8 barrels! Mine shot really really really well

What length did you go with and muzzle device?

My 20" Bison 6.8 Barrel is a tack driver. I own and have built several ARs of different calibers, and I can honestly say the Bison shoots rings around most of them. I like ARP (AR 15 Performance Barrels) a close second.

Tanner is right it shoots great! I have it now. (16") I have also owned a Rainer ultra match and it was unbelievably accurate aswell. From my limited experience & what I have read 16" is all you need. I have had a couple ar-10's and I am not a fan of carrying them around. A light 16" 6.8 ar is the move for me. This will be my third year hunting with the 6.8. I love it! As far of the crown, every 6.8 I have shot or owned has had a "flashsupressor" of some type on it.

What about gas system length? Should I be looking at carbine or mid-length?

From my experiences, well I sold my carbine uppers and replaced them with mid-length uppers in the 223 platform; but that had more to do with the components used then the gas system. In other words, I don't really know. I think I have a preference, but it isn't based on much science.

I am considering this mid-length barrel:

16" 6.8 ARP Scout profile barrel- 4150 CMV
http://www.ar15performance.com/inc/sdetail/11562/14400

I have owned 5 6.8 uppers both 16 and 18
I like my Wilson Combat 16" lightweight best.

I have a Ranier Ultra Match and it shoots very well. I also have a Wilson Combat that has not yet been assembled but I'm quite confident it will shoot well also. The Wilson is fluted and pretty light also. I did have to wait somewhere in the neighborhood of 9 months to recieve the Wilson barrel too so if you're in any kind of hurry they may not be an option for you.

You can't go wrong with arp. Very little advantage in 18" over 16"...like 50fps at most.
My build with arp before optics and paint

Wish I had a better picture but this is my 20" Bison.

Regardless of what anyone says, go look at Daniel Defense Cold Hammer Forged barrels. They are the only 1 company in the USA that has a cold hammer forge and their barrels are amazing. I have an Ambush 6.8 and cannot tell you how impressive it is. I let anyone shoot it, and everyone falls in love with it.

Daniel Defense, Cold Hammer Forged Barrels. Get one.

There may (or may not) be better barrels than the ones Harrison sells at ARP, but I'm absolutely certain that you will never be sorry for buying an ARP barrel. The kind of value they deliver is outstanding. I haven't conducted any chronometer experiments with 6.8 barrels personally so take this with a grain of salt, but the velocity gain beyond 16" in the 6.8 platform is not very significant and drops off rapidly after 18". I think it's debatable whether the velocity increase between 16" and 18" is worthwhile, but the longer barrels will be a little quieter and will probably respond slightly better to a flash hider if you plan on using a night scope. It probably comes down to your aesthetic preferences as much as anything.

You really can't go wrong with a 16" ARP. Mine is a tack driver