OK, Heavy barrel 1 in 9 twist  on the 6721, or lighter barrel 1 in 7 twist on the 6920?????

My first thought is that heavier is better, but anybody have pro's cons on the twist rate of each????

Whoops, hit post again, sorry, Im new.

Heavier the better?  Weight is for artillery, not carbines.

The 6920, hands down.  It's lighter and the 1:7 barrel lets you fire every load out there except the very short varmint rounds.  Easy.

Quoted: Heavier the better?  Weight is for artillery, not carbines. The 6920, hands down.  It's lighter and the 1:7 barrel lets you fire every load out there except the very short varmint rounds.  Easy.

Yeah, but it doesn't say AR15A3 on the reciever. I went with the 6721 'cause I could get it sooner, and I wanted the 1:9 twist. I just wish the barrel were lighter.

another vote for the 6920. the HBAR is just too heavy unless needed for driving in tent pegs.

Phey
I love my LE AR6721.  No, it's not as light as my Sporter, but it's not bad at all.  I'm not going on any 10 mile hikes with it, so what the hell.   hippie.gif  
I happen to like the AR15-A3 rollmarks, also.  Some people will say the rollmarks are not important.  I agree that it has no merit as far as funtion goes.  I like the rollmark because I think it's unlikely that Colt will ever put AR-anything on a rifle that will be sold to the public ever again.
That won't matter a damn to some people; it does to me.

Jim

Two votes for the 6721 based on roll marks, one on the 1:9 twist, but both agree it could be lighter.

Says it all for me.

HBAR is fine.  I'd guess that none of the Fat Bodies on this sight will be patrolling on foot with their rifle for 8 plus hour shifts.  If that were the case I'd say go light weight.  If you can't hold an HBAR up for the amount of time you'll be shooting it, then you should sell the gun and buy a weight set.

I have the 6721, I chose it because the 6920 rollmark is stupid, and the 6721 rollmark is evil.  

I would like to know a place in michiigan that has a 6920 in stock for sale

Joe

Nothing like transition drills to make you appreciate a lighter barrel.

Pretty much the same quandry I'm going through, which makes you wonder why the hell they didnt just make the heavy barrel with a 1 in 7, although colt claims that the 1 in 9 is kosher for both M193 ball and the M855.

The model name is really moot to me, and its not going on any 40 mile hikes, plus I tend to be a fan of surplus Mil spec ball, not reloads or specialty loads, its a toss up, and just may come down to price at this point.

Quoted: Pretty much the same quandry I'm going through, which makes you wonder why the hell they didnt just make the heavy barrel with a 1 in 7, although colt claims that the 1 in 9 is kosher for both M193 ball and the M855. The model name is really moot to me, and its not going on any 40 mile hikes, plus I tend to be a fan of surplus Mil spec ball, not reloads or specialty loads, its a toss up, and just may come down to price at this point.

Well the 1:9 will stabilize the 55grn M193 a little better than 1:7; as well as stabilize M855. The problem is with heavier, longer bullets. If you are sticking with surplus 1:9 is the way to go, IMO.

1/7 will not handle every round available. When using 55gr rounds or lighter weights, the faster rifling can spin the projectile apart. That's why there's a 1/9 twist to slow the spin down for the lighter weight projectiles. The 1/7 twist is ideal to stabilize the heavier rounds.

The HBAR vs lightweight profile dilemma is an old one and subject to fierce debate. I prefer the HBAR for increased rigidity and better thermal dynamics.

One question for the light-weight guys...why shave off metal where explosive and extreme pressures occur, only to add a bunch of heavy free-float tubes/rails, flashlights, lasers and other gimmicks? Let me illustrate my point with a lesson from history: When Charles Lindbergh (you know...the first guy to fly across the Atlantic ocean way back when) was preparing for his historic flight, he tried to find any and every way he could to keep the weight of the aircraft down. In doing so, he even went to the extreme of cutting the edges off all of his maps! A lot of good it did him. On the takeoff roll, the field was so wet from the rainy weather, that he picked up a significant amount of mud on the wheels and undercarriage and almost didn't clear the trees at the end of the field. What did we learn?Some people just can't see the forest for all the trees.

The extreme pressures occur in the chamber, no one shaves metal from there.  

And if someone wants those extra items over the barrel, that's up to them, isn't it?

BTW, Lindberg took off on that wet morning because he was afraid his competitors would take off before him - there was a race for the Orteig Price going on after all - and he didn't have the time to wait for optimal takeoff conditions.  It wasn't his plan from the start to use a wet field, which is the only way your analogy makes any sense.

Now, move along to the grocery aisle, there are some other apples and oranges that need comparing.

I have a 6721 on order from GunDealer3...............There's my vote

Quoted: The extreme pressures occur in the chamber, no one shaves metal from there.

Actually, extreme pressure begins at the chamber and then decreases as the bullet moves down the barrel. The highest barrel pressures exist right where the M4 barrel profile is turned down under the handguards. Look at this chart:
www.armalite.com/library/techNotes/tnote48.htm

I seriously doubt, however, that any semiauto AR15 with an M4 barrel is going to rupture due to pressure, unless you get a barrel obstruction. You'd have to heat the thing up above 1100 degrees before the steel degrades, according to that article.

First why 1 in 7 at all in a carbine?

what I think is this, cross platform use, IE, entry carbine, HB 1 in 9, lots of shots real fast, big barrel, most reliable ammo, and most damaging in a short barrel at close range, M193 ball, and M855 is also very fragmentary at sub 100 meter ranges.

Lighter barrel with 1 in 7, possible use with single shot heavy ammo for "sniper tactical" use, so you have the twist to feed single, long bullet, heavy rounds also, as well as using M855 for entry manuevers.

Im thinking if I want a cross platform rifle, IE, Accuracy beyond 200 meters, go with the 1 in 7, strictly a kick down the door and blast rifle, the 1 in 9 HB.

And seeing that I have a 1 in 7 HBAR full size already for long range use, and a light barreled SP1 carbine with 1 in 9 twist, the decisions been made.

6721, HB entry gun with the flat top for optics, retire the SP, shes tired after being a good rifle, then a Bad rifle, and now turning back into good rifle again, and shes not equipped with a forward assist, poor old girl, practically an antique.

Thanks guys!

Thanks for that article and chart, BB.  I figured the pressure dropped as the bullet moved down the barrel but that spike after the chamber pressure was unexpected.  I appreciate the info!

El Roto,

Okay...I'll bite.

There's no point in arguing with you about the barrel pressure issue. The highest degree of pressure does occur in the chamber. If you don't think there's a tremendous amount of pressure in the barrel, ahead of the chamber, as the barrel pressurizes behind the projectile, until it leaves the barrel, we'll have to agree to disagree? Thanks for proving my analogy correct though (Some people just can't see the forest for all the trees).

Sure, it's everyone's right to add all the weight they want to their weapon. Just pointing out that maybe there's a flaw in logic, if someone touts the weight advantage of a lighter contour barrel and then adds all that weight, and more, back to the carbine. Besides, how many here are going to carry that weapon further than the range and back?

Thanks for the added historical details. Didn't have time to go into a lot of detail and it wasn't necessary to make my point. Boy, I'd hate to sit through you trying to tell a joke. As for the analogy itself, well...it depends on whether or not you got it, which you didn't.

Thanks for the insight though.

Frick,
You get what you like and don't let anyone tell you it was the wrong choice.

Since Im not taking 600 Meter headshots with it, I doubt the faster twist will ever become an issue, like I said, I got along fine for years with my old SP1, but the A1 sights, and non removable top just doesnt allow it to do all the modern optics, it was a good gun in its time, but technology has moved on.

And no, its not for sale, Pre ban is, and always will be, pre ban.

Well, I guess this should have been said already. Can't believe it hasn't.

BUY BOTH (uppers even)

Whatever, john, whatever....

Actually, extreme pressure begins at the chamber and then decreases as the bullet moves down the barrel. The highest barrel pressures exist right where the M4 barrel profile is turned down under the handguards. Look at this chart: www.armalite.com/library/techNotes/tnote48.htm I seriously doubt, however, that any semiauto AR15 with an M4 barrel is going to rupture due to pressure, unless you get a barrel obstruction. You'd have to heat the thing up above 1100 degrees before the steel degrades, according to that article.

Hey, check this out, from that same link. Sounds like some downsides to the M4. Maybe this has all been
mitigated in the newer (or all) M4s, but it sounds like the 6721 may be inherently a bit less "high strung."

(See text below)

-Stooxie

"The carbine gas port is located closer to the chamber than the gas port of the rifle:  7.5 inches instead of the 13 inch distance on the rifle.  The gas pulse therefore enters the gas tube sooner and reaches the carrier group earlier than it does in the rifle length barrel.  In addition to reaching the carrier sooner, it reaches it at higher pressure.  The gas pressure at the carbine’s gas port is double that of the rifle:   26,000 psi vs. 13,000 pounds per square inch.  

The early pressurization of the carrier causes the carbine to begin to extract earlier than the rifle does.  At the same time, the gas in the carbine’s carrier is of higher pressure than it is in the rifle, and it forces the carrier to move the rear at a higher velocity than it moves in the rifle.  Because of the earlier extraction, the cartridge case has less time to transfer heat to the chamber wall and shrink away from it before extraction begins.  The cartridge case has a tendency to stick to the chamber wall, and resistance to rearward movement is high. "

Nope, that problem still exists in the M4, and any carbine-length gas system including the 6721. Only the mid-legth gas systems like those of Armalite and RRA move the gas port itself further from the chamber, and have the added bene of being able to mount a bayonet. What that article is doing is selling Armalites mid length carbines as being better than the M4.