Lightweight barrels are a common topic here.  Lots of requests for recommendations, lots of advice against them because of decreased accuracy, and lots of rumors about the subject.
I went out and tested several of my lightweight rifles for accuracy, and I ask that others do the same and post their results.  For this discussion, lightweight refers to a barrel that is thinner than a government profile or an M4 barrel, particularly in front of the gas block.  I would think 0.625 and 0.750 gas blocks would be fine, as would "pencil" profiles and hybrid styles, and any barrel length.
For my evaluation, I will use 10 shot groups from a bench, using a SS 10X mildot scope in an ADM mount as shown below. The lower used for each upper has a fixed stock and an ACT trigger. Ammunition is commercially available Black Hills 52gr HPBT and Federal Gold Medal Match 69 gr.  Shots were fired at about a twenty second interval.  Each barrel got considerably warm.

Why ten shots and not three or five?  Please read this:  click
Groups will be evaluated using the On Target software available for free online and both Extreme Spread (CTC, center to center) and Mean Radius (ATC, average to center) will be provided.  For some reading on these ideas go here: click
The most important measure of the center is the mean radius.  It tells how all of the shots in the group compare to each other.  Extreme spread only gives the distance between the two farthest shots; it does not tell anything about the other shots in the group.  For example, a mean radius of 1" means that half of the shots fell within a circle with a 2" diameter.  Ammunition and a rifle that produces a mean radius of a little under 0.3" is considered match quality.
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1. First rifle is built up using a factory assembled BCM BHF hammer forged 16 inch 1/7 5.56 midlength pencil chrome lined barrel.  I have previously tested this barrel as can be seen here: click








The first six rounds of the GMM went into that little cluster with one thrown right.  Then the range officer called "one minute" and I lost it in rushing the last four.
Note that the top group has a SMALLER extreme spread (labeled MAX, the straight line distance shown) than the bottom group, but a LARGER mean radius (labeled ATC, shown as the circle). Think about what that means.
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2. the second rifle is a home assembled RRA 16 inch midlength 1/9 5.56 barrel with chrome lining and a 1/9 twist.  This barrel is a bit thicker than a pencil barrel, measuring about 0.65 in front and behind the 0.75 gasblock.  I have previously tested this barrel here: click


First group went really well.  But the GMM was a fight.  I couldn't seem to get the rifle settled and was dealing with some glare off the front sight ears.  It is what it is.  
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3. The third rifle is a home assembled Daniel Defense 14.5 inch midlength 1/7 5.56 barrel with chrome lining and a pinned C2 Phantom flash hider.


On the top group, I forgot that I had a FMJ hand-charged round first out of the mag and I put it on the target instead of the dirt.  It was not included in the group.
I was surprised at how low the 69 grain shot compared to the 52 grain.  On the two 16" rifles, the heavier load was about an inch and a half low.  The 14.5" barrel shot them 3 inches low, almost off the paper.  I did not expect to lose that much velocity with 1.5" of barrel.
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Conclusions:  All rifles have standard hand guards, are not free floated, and all rifles shot well within my personal defensive standard for accuracy:  A Mean Radius of 0.8" or less and an Extreme Spread of 2.5" or less.  This standard implies that at 500 yards, half of the shots will fall within a 8-inch diameter circle, with none exceeding 13 inches.  The width of my torso is 13" and I've got a medium athletic build.  
I cannot imagine a scenario short of TEOTWAWKI that would require me to make a rifle shot at a distance greater than 100 yards.
A lightweight barrel of good quality with standard hand guards is just fine for a defensive long gun.
and as a homage, here's Molon's test of a Colt LW barrel click
 

How far did you shoot? That looks like a really short range.

 

Here's some results from an M16A1 1/12" barreled upper I shot a while back





Test bed, not free floated:






Hornady 52gr HPBT, Extreme Spread of 1.543" ,  Mean Radius of 0.472"






And on another day, 70 Degrees and very breezy- wind gusts up to 20 MPH, each group has a few stray rounds.
Ammunition from top to bottom, all handloads
52 Sierra HPBT



52 Sierra HPBT



55 Hornady FMJBT


 

excellent!  thanks!



Is the omega rail a free float?  I'm going to take my 16" 1/9 bushmaster superlight out for a run this weekend and see how it compares to your Colt.



and not to bust your chops, but I only count nine shots in your first group.



 

I would think the stock on your rifle makes it difficult.  The telestock probably does not settle into a rear bag very well.  I do all of my testing with a fixed A1/A2 stock and a rear bag with ears.  





PS, the way your scope sits right behind your front sight base makes me chuckle.  Mine does the same if you notice.  It sure looks silly, but it works.

 

New Results Today:





I took my home-assembled old-Maine Bushmaster Superlight 16" and shot the same ammo as previously, Black Hills 52 gr HPBT and Federal Gold Medal Match 69 gr HPBT.  This upper has a pencil profile, carbine gas, standard handguards, 556 chamber and a 1/9 twist.  I used my fixed stock lower with the ACT Trigger and SS 10x mildot in an ADM mount.





Upper:









I shanked a round low in the 52 group, but the 69 load really surprised me.  I've never used this upper for anything other than blasting.  The barrel was quite hot by the time to got to the end of the 69 string.





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I also shot my BCM 16" Midlength with some Federal Fusion MSR 62 grain SP and some Wolf Gold 55 FMJ.  Upper also has a pencil profile, midlength, 556, and 1/7 twist.





upper:




Again, I pulled a shot low in the top group, and the Wolf Gold surprised me.  I expected at least a 3 inch extreme spread.

 

Bump after the move.  I doubt it would get much traffic in here.

bump- please add your shooting results!

I'd love to see those results.  I have been looking at that barrel for some time



 

Good show on the Fusion.  I've got a bunch of those to load up and hope they shoot that well



 

well, I keep my crap groups private

bump, anybody have new data to add?

Shot the M16A1 barreled upper again yesterday



















 

I shot my BCM 16" Lightweight Middy again.  100 yards, 10 shots, 10X optic (Not the ACOG)












Black Hills 69 and 77grain
















Was doing well with the 69, first seven shots went into a nice group.  then a fly bit me on the heel of the hand when I was shooting the 7th shot and after the battle the last three were down low.  


And the 77 was fine but the last shot went way wide.  I dunno.