Building a varmit upper. Flat top upper, freefloat tube handguard. I have a Hart 1-7 twist heavy barrel blank to use. Looking for extreme accuracy. Any suggestions for finished barrel length appreciated. I'm thinking 24 inch but havn't decided.

Accuracy and barrel length are not related.  Get whatever length you like best, a Hart is a Hart and it WILL shoot.  

If it matters to you, it is easy to wring out more velocity out of heavy bullets using longer barrels.  A 24" barrel can get nearly 2900 fps from a Sierra 80 Match King with a healthy-but-safe dose of Vihta Vuori N550.

You'll get some really in depth answers on this one, but I'll start off with just a little education for you.
Longer barrels DO NOT equal better accuracy bro.
Longer barrels Offer better velocity.
A 16" barrel can be just as precise as a 24"barrel. Some will give you facts about shorter barrels actually being more accurate due to them being stiffer with better harmonics. A longer barrel will give you higher velocity which will give you a flatter trajectory and give you a little more punch. I used to think the same thing as many do, but it ain't so. Now being that your building a varmit upper, You probably want a 24" so you have better knock down power at longer ranges, I understand, but just to let you know, Some Guys are doing wonderful things in the W.O.T. with 18" SPRs in Iraq and Asscrackistan.  Good luck,
chuck

While I must agree with eaglecp on barrel length not necessarily equaling accuracy, I would say that more than adequate accuracy and velocity can only come from a longer barrel.   Space guns can be 26 or 28 inch wonders looking for 600 and 1,000 yard accuracy, but if the caliber is .223 reality sets in faster than a .338 Lapua.  I shoot a Colt accurized rifle 24" and it's a 1 in 9 twist which has taken Pdogs at 1,106 yards with handloads.  Generally, it's fine to 600 yards and 100 yard groups (once again with tailored handloads) are in the .3 MOA range. Factory rifle and 24" barrel with a Jewel trigger and a Leupold Mark 4 - 16X.  Wind is the issue and speed is the only cure for wind.  Speed can only come from a longer barrel.  Bottom line is in .223 I would not trade or change my 24" Colt for varmint hunting.  The two best bullets for me have been the Nosler 55 BT and the Hornady 75 A MAX.  Good luck and great shooting.  PS if you have a Hart catalog the cover rifle has been a .308 Sako with a rather nice whitetail... guess who's!

Quoted: While I must agree with eaglecp on barrel length not necessarily equaling accuracy, I would say that more than adequate accuracy and velocity can only come from a longer barrel.   Space guns can be 26 or 28 inch wonders looking for 600 and 1,000 yard accuracy, but if the caliber is .223 reality sets in faster than a .338 Lapua.  I shoot a Colt accurized rifle 24" and it's a 1 in 9 twist which has taken Pdogs at 1,106 yards with handloads.  Generally, it's fine to 600 yards and 100 yard groups (once again with tailored handloads) are in the .3 MOA range. Factory rifle and 24" barrel with a Jewel trigger and a Leupold Mark 4 - 16X.  Wind is the issue and speed is the only cure for wind.  Speed can only come from a longer barrel.  Bottom line is in .223 I would not trade or change my 24" Colt for varmint hunting.  The two best bullets for me have been the Nosler 55 BT and the Hornady 75 A MAX.  Good luck and great shooting.  PS if you have a Hart catalog the cover rifle has been a .308 Sako with a rather nice whitetail... guess who's!

listen to this guy...

+1

you need longer barrel for the round to be effective at long range.  the faster the bullet, the less wind will efect it.  thats why i like 26 inch barrel .308's.

for an AR, i would use anywhere from 20 to 24, no more no less...

What ranges do you plan on shooting your varmints at is the question you need to consider. Do you plan on adding a suppressor? If the thought has ever crossed your mind, make sure you get the barrel right the first time, so you don't have to have it turned down later for mounting options.

I've had 100 meter groups, 16 rounds was the best group, that was less than 1moa out of my 20" gun. That was with factory federal gold match 69 gr from a 1/8 bull.

I suppose that the extra 4" would give you a noticeable velocity change, but what it equates to at extended ranges is only about another 100 meters (by my math) in the supersonic range.  

+1 to the 18" SPR stuff downrange. They work, without a doubt.

If you want to shoot really far 600 yards plus you might be better served by a longer barrel.  As this will keep the velocity higher and the faster the bullet gets there the less time it has to be effected by outside factors, like wind.  That being said a particular rifle might shoot a particular round better at any distance at a slower velocity.  It is quite possible that the most accurate load in your 24" barreled AR might be at the same velocity as the guy next to you with a 16" barreled rifle.  If accuracy is your primary goal and not terminal effects I would go with simply whatever barrel you feel most comfortable with.  If you are looking for a balance of terminal effects and accuracy willing to give a little accuracy for the terminal effects at long range I would go with the longer barrel.  YMMV and this is strictly personal opinion from my limited experience.

Quoted: What ranges do you plan on shooting your varmints at is the question you need to consider. Do you plan on adding a suppressor? If the thought has ever crossed your mind, make sure you get the barrel right the first time, so you don't have to have it turned down later for mounting options. I've had 100 meter groups, 16 rounds was the best group, that was less than 1moa out of my 20" gun. That was with factory federal gold match 69 gr from a 1/8 bull. I suppose that the extra 4" would give you a noticeable velocity change, but what it equates to at extended ranges is only about another 100 meters (by my math) in the supersonic range.   +1 to the 18" SPR stuff downrange. They work, without a doubt.

If you don't mind, could you elaborate on the suppressor comment? I've got an AAC M4-2000 on order and I'm looking for another host platform. I was thinking of a WOA 18" with flutes and all the goodies. Accuracy and portability are paramount with this build. Any input would be appreciated.