Dissipator Uppers for AR-15 (what are they "dissipating"?)

[rantingredneck]

I'm new to the whole AR15 thing. I own a 16" carbine and am in the process of putting together a 20" rifle.

A question struck me though as I was logging in to TFL. I saw the graphic with the Dissipator AR and it got me thinking, What's the purpose of that configuration and what exactly is it Dissipating?

Just a random question for the day..........

[teeroux]

Its usually a short barreled variant with full length rifle gas system and hand guards. Its supposed to give you the shorter barrel with the benefit of the lowerer presure, easier on the action, full length gas system.

[RT]

I think the point is to have a shorter barrel with rifle length sight radius. Usually these have a lo profile carbine gas block under the hand guards. CMMG offers dissipator uppers with carbine, midlength, or rifle gas system.
http://cmmginc.secure-mall.com/shop/?cart=1585835&cat=27&
I am not exactly sure what it is "dissipating".

[Technosavant]

I guess they're "dissipating" bad guys.

As said, the Bushmaster Dissipator does NOT have a rifle length gas system- it has a carbine length low profile gas block under the handguards with a regular rifle length handguard and sight radius (the front sight/gas block isn't used as a gas block). They look a little weird, but I guess if you plan to use irons or you just want acres of handguard space they might be worthwhile (the same thing can be done with a regular carbine or midlength 16" barrel, a low profile gas block, and 12" railed free float handguard, complete with handguard mounted front sight).

[AK103K]

I have an early Bushmaster Dissipator. I bought it because at the time, the only other 16" barreled guns were the silly looking guns with the pencil barrels, very short handguard and sights. Basically, just like the HK 94's and UZI's, compliant and legal barrel length copies of rifles that would otherwise be in the NFA realm.

While I like the longer sight radius and longer handguard, I really dislike the heavy barrel they put on their Dissaparors, which make the gun needlessly heavy and a little ungainly.

The longer sight radius did nothing for accuracy, or at least for my gun, as its the most inaccurate AR I own.

Thankfully, the makers changed their ways and came out with the "Carbine" length handguard rifles, and a more balanced looking and handling rifle emerged.

[TheManHimself]

It's not "dissipating" anything. If anything, having more barrel covered by the handguards would make it take longer for the barrel to cool, due to the heatshields inside the handguards and reduced airflow over the barrel.

[rantingredneck]

^ I kinda thought that myself but wondered if I was missing something.

Thanks for the replies.

As I said, just a random thought that struck me as I was logging in today. I've never liked the looks of the Dissipator style uppers and have no intention on purchasing one.

[JDTech]

Adding a longer handguard, especially made of an alloy like aluminum is like wrapping a sponge around your arm if it were sweating. More surface area equals more cooling. The aluminum pulls the heat out of the barrel. Same principal as an air cooled engine. If they didn't stack all those extra air vanes all over the cylinder heads they would overheat. They hand guard has a much larger diameter plus much more surface area alloying more air to touch it as it passes by pulling even more heat away.


Hence, diassipating heat!

[ATW525]

As JDTech said, the longer handguards are dissipating heat. Short carbine handguards don't do it as efficiently, which is why the USGI handguards for the M4 are oval shaped with double heat shields to help compensate.