Well sir.......that does leave a few more questions to respective replies. These days assuming things.....well....... Anyway, I just have to ask. Caliber? These days AR15s come in many varieties, several calibers. (I am assuming 5.56/.223 here and will explain my theories and personal experiences with these later). Match grade bullet? Please explain further. Velocity, accuracy, and distance....expectations?
Here's my take on the AR15 in 5.56/.223
The USMC handed me a 20" barreled M16, standard milspec ammo, and expected me to perform well with that on target out to 500yds from a variety of firing positions with open sights. This led me to think of the cartridge and rifle being somewhat capable of doing the intended job out to a max effective distance of 500yds. Punching paper or dropping steel is a totally different task than....well, you know. Eight months later they gave me a bolt action .308 and expected way more. Accuracy? The usual when it comes to caliber, chamber, and shooter. Then there is rifling twist and bullet weight. I find 5.56 ammo more accurate in my AR15 and my rifle sports an 11.5" 1/9 twist barrel specifically chambered in 5.56. I do use .223 ammo for general range use since it's less expensive. I don't expect my max effective range to be more than 300yds, and this baby is well suited for 3-gun matches. Accuracy with .223 ammo in a 5.56 chamber isn't that much different until you get past 200yds. Distance is actually related to purpose. If you want to hear a ring off steel at 600yds, everything is cool. That can be easily done with the caliber and rifle. If you custom load for a semi-auto, then I figure you can increase the odds in your favor just like you can with a bolt action however may be more limited in ways.
So.....this is just my quick take on your general question here. Some may also disagree with what I've said, but remember, it's all from my personal experience, my thoughts, and some outside influences that I've come across over the years.
One thing I can tell you for sure, and some will likely agree........four extra inches could get you more options in life alone, forget about the rifle. Needless to say, I'm not a movie star. It is what it is.

 

It depends on the bullet, but plan on about 125 fps more with the 20" barrel.

Accuracy really depends on the barrel more than the barrel length. The average Colt 1-12" A1/SP1 style barrel would deliver accuracy around 1.25-1.5 MOA. The average 16" lightweight carbine barrel is closer to 2-3 MOA.

Short, heavy barrels however can be very accurate. I have a 16" bull barrel that will shoot sub MOA with match bullets, but i also have a 20" bull barrel that will produce half MOA groups with match ammo.

My compass lake 1-8" twist heavy match barrel will shoot about 3/4 MOA.

One thing I've noted is that the standard deviation in velocity is generally lower with a longer barrel and that helps reduce dispersion at long range.

In short, if your primary concern is accuracy and longer range shooting, you'll be better served with a 20" barrel. short barrels are the current fad given the recent unpleasantness in Iraq and a focus on close quarter battle, but there is still a lot to be said for a 20" barrel. I'm biased by what I was issues way back in the day, but I still like the weight and balance of the 20" A1 profile barrel. The A2 adds a half pound of weight between barrel and hand guards and the handling qualities suffered. And bull barrels, while they can be remarkably accurate, are pretty heavy and are not something you want to carry all day.

 

There should be tables...

How much more velocity, accuracy and distance could I expect using a 20" barrel over a 16" barrel with a good match grade bullet?

There should be ballistics tables for velocity and drop between the two differing barrel lengths. The common muzzle velocity of issue ammo is said to be 3250 fps, more or less. Gotta be a chart somewhere telling us about what would be the muzzle velocity of that same ammo fired from a 16" barrel. Once we know the difference, we can just drop the velocities and masses of the bullets into a ballistic calculator and push "Calculate." I'm a member on another site that has such a calculator. I can just put in 3250 fps, 55 grains and 1,000 yards, push the button and out will come a plot of the trajectory along with velocities at every fifty yards, let us say. Put in the lesser velocity from the shorter barrel, push the button and out comes that trajectory. As far as accuracy, the calculator won't do that beyond what amount of deflection you'll get per a given wind velocity and direction.

I once cobbled together a load and fired it from a 16" barrel. I got right around 2750 fps with 55-grain FMJs. Might have been 2650. Was five or six years ago. Don't rightly remember. We'll say 2700. So the issue fodder is approx 3250; my load was approx 2700. That's about 500 fps in the four-inch difference. Didn't somebody above opine that four inches would yield about 500 fps difference?

 

We have added the option of a heavy match grade barrel, and another veteran's take on the old platform the way it was basically designed. It did balance well especially for off hand shooting. Not sure what another 125fps might get you out on the streets these days, but back when things were more important it was accurate enough.

 

Check your PMs, silky...

 

Thanks guys... Sorry for the vague question.. I think I am getting way too far ahead of myself. I have an AR15 5.56 with an inexpensive 16" 1-7 twist non chrome lined barrel and a standard mil spec trigger that I assembled from parts just for plinking. I shoot federal 5.56 62gr green tips that I get from Wally World. It's a good shooter but I had so much fun researching ,ordering parts and putting it together that I would like to build another 5.56 with the intention of longer range accuracy meaning better barrel, trigger , a 3x9 scope and using some match grade ammunition. (Not even sure there's a range to shoot longer distance) and was just wondering if 4" of barrel from the same manufacturer would make that much of a difference.

 

Id go for the 4" of barrel!! Why--cause. You already have a 16", sure dont need another. Go for a 1:8 or 1: 7 on the 20" er so you can shoot heavier bullets.

 

There are things to be said about a chrome lined barrel over a standard steel barrel as well. Especially when it comes to cleaning and velocity (why I like my chrome lined barrel). Just there are so many variables to consider......not that you were vague with the question. All things being equal, I doubt you'll see much more out of a 20" barrel than you're getting from a 16" barrel of the same type from same manufacturer. 5.56 chamber should see best results from 5.56 ammo for sure (if you can find and afford to shoot). Your barrel twist rate seems to do well with the heavier bullet weights up to 77gr. which can easily take medium game with the right bullet at decent distance. Building your own? There's a certain satisfaction from doing that for sure. A good shooter is a good shooter. That's primarily the reason we keep the good shooters and try another build in order to improve upon the past. A good shooter is like having a winning horse in the stable. It's just we seem to want more and a way to improve upon what we've done. That's only human. At this point I'm going to suggest that using match grade ammunition in your AR15 will eventually eat you alive in cost for ammo alone. You're talking inexpensive when it comes to rifle, but expensive when it comes to ammo. Those two don't mix or fit into the same equation in my book.
Look up some articles on high power and David Tubb on the almighty internet. The 5.56/.223 ordeal, chrome lined vs steel....etc. Plenty of good reading, learning, etc.
You've already done more than most folks would as far as building your own. Keep the budget in mind, and make it the best you can by all means. You have my best wishes.

 

The company I bought my barrel from said they use Bergara barrels from Spain which I have read is very good steel.. Before I build another rifle I might upgrade my trigger and try some 77gr match ammo. Maybe get some .223 dies and start reloading to cut the cost.

 

Maybe get some .223 dies and start reloading to cut the cost.

Just make sure you get small-base dies. I like RCBS for such. The small-base type sizes the brass to make dam-ned sure it will chamber. The AR15-type rifle does not have the ability to "cam in" a round like does a bolt-action gun. Get yourself an L. E. Wilson case gauge. This little doo-dad will tell you if the case is correctly sized. I learned far too late about this thing. They're cheap when compared to having to break down and rework a ton of badly-sized ammo that won't fully chamber and lock in. Have the shoulder just a few thousandths too high, and it's No-go City. Believe me; I know...

 

Up there somewhere it was said that the 4" extra on the barrel would get you 500 fps... all else equal.. NEVER.

150 fps would probably be a stretch for 4 more inches of barrel-all else equal.

 

I have a 16" barreled RRA LAR-15 and I get about 2700-2800fps with win 748 under a 52gr speer. I get 3200fps with AA2230 under a 60gr Nosler. So, velocity can vary quite a lot from the same barrel depending on the powder used.

Personally, I like the 16" barrel for hunting deer and tactical fun.

My RRA LAR-8 in 308 wears a 20" barrel and it will shoot just as fast as any bolt gun with the same length barrel. 2500-2800fps is the norm depending on bullet and powder used. The tradeoff is the longer barrel weighs more and is harder to maneuver in the woods and tight places.

I must add one thing too. None of my barrels are chrome lined and I would NOT want chrome in my barrels. NO ONE who makes a match grade bolt action barrel uses chrome lined barrels, and I see no reason to treat AR type rifles differently. We are not fighting in the jungles and burning thru 1000's of rounds of ammo in a few hours. My RRA barrels are also dang near impossible to get any copper fouling in them and they clean up with a patch or two and that's it.

 

As someone else said 100-125 fps increase-max-all else equal.

 

Here is a decent read on the subject.
223 Remington/5.56 NATO, velocity versus barrel length: A man, his chop box and his friend’s rifle | Rifleshooter.com

Going from 16" to a 20" barrel, velocIty increase a little over 100 fps, standard deviation drops, and felt recoil drops slightly but noticeably.... weight of barrel, ejecta....

 

I have a Colt A3 with Wilson 16" wylde BB and I do not think the 4" difference makes any difference, I shoot a 3 shot group @ .57" at 100yds. It is an excellent setup, btw this is with steel case ammo

 

Silky320, a link to read some time. http://www.thetruthaboutguns.com/20...10/daniel-zimmerman/the-truth-about-barrel-length-muzzle-velocity-and-accuracy/
Also, with reloading you can tailor the bullet that the barrel "likes" and work within the harmonics of the barrel. I've always contended that there exists excellent commercial loads and they can't hold a candle to excellent well developed hand loads.