[DMK]

It would seem to me that a longer sight radius would be advantagous. Why do they put the front sight halfway back the barrel?

http://www.bushmaster.com/shopping/weapons/Images/PCWA2S16.gif

[ether]

The first carbines had very short barrels (10") where the muzzle was just past the front sight. A standard for carbine-length handguards was established, and civilians that want a "carbine" have to settle for a 16" barrel. Carbines are handier and lighter than even a 16" Dissipator-style AR. They point faster, and the shorter sight radius makes sight acquisition faster.

[bad_dad_brad]

Has to do with gas pressures, that is, where the gas port has to be to be effective at operating the bolt properly. Most 16" AR15 carbines look like the picture posted, the Bushy Shorty configuration. Personally I think this is the best over all AR15.

The Bushy Dissapator has the exact same gas port. They just fool you by having a 20" type handguard to extend the sight radius. Underneath the handguard, the port is positioned the same as a Shorty.

I think the Dissapator is nose heavy and appreciate the handiness of the plain 16" barrel. Sight radius is not that big a factor for a carbine chosen for close range work.

[boing]

Tradition, as ether said. The original carbines had handguards proportional to the barrel length, so the civilian versions were just copied that way. Note the M4 is similarly proportioned, but has a more evenly balanced appearance because of the flash hider.

RRA makes a 16" carbine with "mid-length" handguards:

http://rockriverarms.com/images/tasc.gif

It looks well-proportioned, IMO, but with the unusual front sight location, I would think the elevation dial on an A2 sight wouldn't be properly calibrated. But maybe that doesn't matter on a CQB gun, anyway.

[john l]

I actually got the RRA with the mid length handguards, and I am impressed with the slightly longer sight radius without the dissipator look, which to me looks rediculous(just my opinion)
I also got a birdcage muzzle brake on it mostly because it is one of my 3 gun match rifles and protecting the crown is a good idea too.
I am totally sold on the mid length handguards.
John L

[COHIBA]

the better to poke you w/ my dear.

[DMK]

I like that mid-length handguard better too. It looks more asthetically balanced to me, kinda like a baby A2. I've never handled an AR carbine though so my opinion on that matter's probably so much fooie.

The Bushy Dissapator has the exact same gas port. They just fool you by having a 20" type handguard to extend the sight radius. Underneath the handguard, the port is positioned the same as a Shorty. I'm still not sure I understand why the gas block needs to be so far back though. If a 20" barrel has the gas block at X inches, why does the 16" barrel need it so much further back with the same ammo?

[Navy joe]

As Brad pointed out, it's the gas system. With the AR the gas porting is part of the front sight tower. The shorter bbls. need the port farther back to generate sufficient operating pressure since once the bullet leaves the barrel pressure drops to nothing. The effective legth the gas system has to operate is from the time the base of the bullet passes the port to the time it leaves the muzzle.

[DMK]

The effective length the gas system has to operate is from the time the base of the bullet passes the port to the time it leaves the muzzle. Ah, I see. [lightbulb clicks on above head] Thanks!

[Jim K]

The gas port on those guns is fairly far back because a high pressure is needed to operate a gas tube system, especially with a round like the 5.56mm, since a lot of pressure dissipates in the tube.

There is no inherent reason for locating the front sight on the gas port block, but that location makes manufacture and production easier and cheaper. Further, placing the sight at the end of the barrel would make it even higher and give it less support, thus making it more prone to damage.

The sights are high on the AR-15 type rifles for the same reason they are on other rifles originally designed for full auto. To reduce climb, the designer uses a straight line stock. Then, to allow the rifle to be held normally, has to use a pistol grip and high sights. This then leads to high and fragile sights. The carrying handle is simply a matter of making lemonade from lemons; it makes use of the space required by the high sights.

Jim

[Chipperman]

I have a Dissipator that I fired for the first time the other day. I am very pleased with it. It is sl more nose heavy than a shorty, but not enough to bother me.
I appreciated the longer sight radius with the same length barrel. I also don't like blast enhancers, which the Dissy does not have.