The Dissipator concept came about briefly during America’s involvement in Vietnam. Early slab-sided Colt 601 AR-15s, Colt 602 XM-16s, Colt 604 M16s, and forward-assist equipped Colt 603 M16A1s all had full-length 20-inch barrels with a good portion of that being past the gas block/forward sight assembly.
![](https://laststandonzombieisland.com/wp-content/uploads/2023/12/1962-rifle-automatic-caliber-.223-armalite-ar15-colt-model-spar-30-62.a.1.jpg)
A circa-1962 layout from the U.S. Army’s Springfield Armory showing an early Colt 601. Note that the last 6 inches of its 20-inch barrel are past the forward sight assembly, with overall length hitting just shy of 39 inches.
Troops in tight situations, such as crammed into helicopters and pushing through triple canopy jungle, found themselves wanting something handier. This led to a “field modification,” which saw some simply hacksaw a few inches off the barrel to make the rifle shorter. This mod even became semi-official, with in-country workshops whittling down the barrel a couple of inches and then threading the muzzle to attach the flash hider.
The hacked Dissipator was born – although there is no evidence that it was ever called this in military service. The mod made the 6.5-pound early M16 more compatible in overall length to the 35-inch long M1 Carbines which, while not in U.S. frontline service at the time, were often “acquired” from local South Vietnamese troops due to their ease of carry.
The unwelcome news was that the Dissipator mod killed the rifle’s dwell time and made a gun that was already of questionable reliability at the time even more prone to fail. Doh.
![](https://laststandonzombieisland.com/wp-content/uploads/2023/12/dissy-vietnam.jpg)
Left is a workshop wall at the U.S. Army’s Springfield Armory with a Dissipator M16 second from the top. Its formal industry replacement, the Colt 610/XM177, can be seen above the Dissipator on the wall. Right is a Fleet Marine Force M16 workshop in South Vietnam apparently reworking guns to make them shorter ala Dissipator style, in April 1968. (Photos: Springfield Armory National Historic Site/National Archives.)
Colt even made the Dissipator concept refined with the Model 605, which included a bayonet-lug-less 15-inch barrel whose flash hider started right where the front sight assembly ended and a full-sized fixed buttstock and a rifle-length gas system. However, it was very soon superseded by the Colt 610/XM177/GAU-5A, which entered service by the late 1960s. With its adjustable two-position buttstock and a 10-inch barrel with a carbine-length gas system, it only took up about 28 inches of real estate and would go on to be the go-to shorty M16 for generations.
While the military walked away from the Model 605, it turned out the concept of an AR-15 platform with a full-length fixed buttstock and handguards with a rifle-length gas system on a trimmed-down barrel made for a smooth-shooting rifle while still coming in (a little) shorter than a traditional 20-inch full-length rifle.
A niche for sure, but one that black rifle makers took a chance on over the years with Adams Arms, ASA, Bushmaster, Delton, Doublestar, KAK, PSA, and Windham Weaponry all selling their own assorted takes on a Dissipator for the commercial market. The thing is, most of these are “mock” Dissipators, as they actually used carbine or mid-length gas systems with a low-profile gas block under the handguard. The A2 sight was pushed out as far as possible to give the short look of the Dissy while getting away from the old dwell time issue the Vietnam-era guns suffered from.
![](https://laststandonzombieisland.com/wp-content/uploads/2023/12/faux-dissipator-top-mid-length-16in-2nd-is-standard-20in-rifle-length-pencil-bottom-true-dissipator-rifle-length-system-16in-hbar.jpg)
Faux Dissipator: top mid length 16inch, 2nd is a standard 20-inch rifle length pencil, bottom is a true dissipator with a rifle length system on a 16in HBAR
The latest Dissipator comes from Anderson, of all people, and they got the dwell time right without resorting to faux-ing it up.
I’ve got 500 rounds through it thus far and I think I am falling in love.
![](https://laststandonzombieisland.com/wp-content/uploads/2023/12/dsc_0436.jpg)
The recoil impulse on this thing is smooth. I just wish it had a carry handle upper
More in my column at Guns.com.