I am interested in building a 20" medium/light AR15 upper.  I want a easy carrying scoped varmint rifle this time.  I will use a free-float tube, probably aluminum with plenty of holes for ventilation.  It will be on a flat-top upper. Since it is not for 'combat use' I have no need for backup sights or lights, etc., just a good medium power variable scope (3-9 or 4-12x or similar).

For a barrel I DO NOT want a big bull barrel, I want this entire rifle to be light yet accurate.  I have heard the thin "pencil" barrels shift point of aim after 10 or so rounds have been fired and that a fluted mid weight barrel is the way to go as it is much more stable when heating up and weighs little more than the old "A1 government" contour barrel.

Can anyone out there verify this?

You might try a 20" Colt A2 barrel. They are accurate and lighter then a fluted HBAR.

thanks for the tip nvcdl, I will be scouting for that barrel.

My first thought was an A2 barrel too, but they're as thin as the pencil barrels under the handguards so I don't know how they'd handle the heat.  They are lighter than a fluted HBAR though.  (IIRC 3.5lb for an HBAR, 3.0 for fluted HBAR and 2.5 for an A2).  

If you want a little bit more weight savings w/o going all the way to an A2 I suppose you could have an HBAR turned down to something inbetween, maybe .75" (HBAR is 0.9-1.0 and A2 is .63 under the HGs).  That could be fluted too.  Might get close to an A2 weight then, and would be a little stiffer.

Edited to add: Bushmaster makes A2 barrels too.  Probably cheaper than the Colt (isn't everything?) and 1/9 twist, which might be better depending on what ammo you had in mind.

I haven't noticed significant opening of my groups as the A2 barrel gets hot, however I uses a Medesa free float tube. The thinner barrels are going to be effected more if they are not free floated.

The A2 design with the heavier .750 profile at the muzzle may act to stabalize muzzle whip.  

BillSP1,

I'm in the middle of building the same type rifle except with a fixed carry handle and match sights.

Now that I have the lower just the way I want it, I will have the Hbar turned down to A2 profile and have a lightweight forearm installed.

Think I'm going to try the KA grip tube. It weighs just 10 ounces and has plenty of large holes for cooling. I too won't have much attached to the forearm except a bipod stud.

What kind of free float tubes are you considering?

I personally like the smaller, slightly lighter 1X4 or 1X5 power scopes for the hunting AR but the 3X9 is also a great all around scope. I have a 3X9 I use to test ammo.

Matt  

ARMatt:
Not sure on the optics for this project, but as I intend to use it as a "walking varmint rifle" I want to be able to take a p.dog head shot at 300 yards absolute max.  This pretty much means I need to see a 2 inch target at 300 yards, so some decent magnification is called for.  I may use a 2-7X to keep the optics compact and cost under control.  I am a Leupold fan, but have been admiring the Bushnell Elite scoped lately as well.  Probably don't need mil dots or target turrets on this one.

Whenever I start thinking about higher magnification I have to remind myself of all the great shooting I did with my first 223 fitted with a fixed 4 power Burris compact scope.

On the free float tube I will probably use an aluminum one and have to drilled out profusely (or slots milled) to allow better barrel cooling and reduce weight about an ounce.  The Armalite fiberglass tube is also appealing.  It is so damned hot here in the desert that the sun alone makes rifles untouchable for the next few months (105-115 for a few months now).