I understand that the term dissipator refers to a special gas block that is bfore the front sight.  The front sight is positioned all the way forward for better sight radius.  What is the reasoning for this setup?  Does the gas block need to be a determined length back from the muzzle for everything to cycle properly? Or is there some other special proprty of the "Dissipator" model?

From my understanding, with a Dissipator, you get the benefits of the longer sight radius with the long handguards and the, from what I've heard, better gas setup from the midlength placements of the gas block.

I remember reading somewhere that the mid-length gas systems are optimal for the AR15 system but I do not think that there is any "special" place for the gas block to be located.  I think it has something to do with how much gas is used to force back the carrier after a shot is fired.

Hopefully, someone more knowledgeable will chime in.  I am curious as to why a mid-length setup is better.

The BM Dissipator has a carbine length gas system.
There is a very important time factor in the gas system, that comes from the block to muzzle length.
As far as I know the car gas system is the best bet.

BTW, I got a chance to handle a new BM Dissipator, and WOW!  I'm in love!  The Dissipator will be my next rifle(maybe a Colt mod, for the 1/7)..[coffee]

just get a standard 20 inch and have KKF custom fit a car gas system to it. got sight radius of disappator, car gas system and still retain your bayonet facilities.

What do you use your bayonet for... hot dog roasts?  [:D]

AWB aside, would you prefer bayonet lug, or would you not care if you had one if AWB was no longer in existence?  It seems the reason most folks want it is becasue they can't have it. (my opinion)  Don't hear about a lot of serial bayonettings out there.  Not very efficient tool.

Don't care.

As for the dissipator, the system makes sence, if the looks don't bother you.

I love the way the Dissipator looks.

I think it’s a great compromise of the 20-inch and M4 (was there ever a compromise called for?) Mid length gas block, shorter barrel, full length sight radius.

I think they all should have a bayonet lug, it just looks right.

KKF can make you one with a 16 inch barrel and the gas port in the rifle position.

I love mine.  It "feels" a little nose heavy, but If I had a "standard" 16" barrel of the same profile, it would probably feel the same.

I really like my Dissy, although I had the barrel turned down and lightened.  

The HBAR version is too heavy IMO.  Bushmaster is now making a M4 profile Dissy barrel that will be lighter.  

The relationship between the distance from chamber to gas port, distance from the gas port to muzzle, and size of the gas port is a complex one.  Changing any one thing (for example by cutting 4" off the end of the barrel) without changing the others will likely end up with the rifle not functioning.  Bushmaster's solution to that problem on the Dissapator was to place a standard 16" gas system under the HGs, so it works just like any other 16" rifle.  The other solution, as used by KKF and AFAIK all of the other "dissapator" style rifles, is to leave the gas port at the 20" location and adjust its size.

According to Armalite, the "mid-length" gas port placement is more reliable on a 16" gun than the regular CAR location, because the pressure has dropped off a bit.  I've seen some reports that the 20" M16s are more reliable than the 14.5" M4s in military testing, but nothing including the 16" mid-lengths.