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Centerfire Rifles - Semiautomatic or Gas Operated Centerfire rifles, carbines and other gas operated rifles. |
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Calguns Everything about the Dissipator thread. pics, discussion, tech
This is a thread for discussion and sharing of the Dissipator type Ar-15 One place for information, history, and photos! For the purpose of this thread, the term "dissipator" will refer to the design concept and not the specific model name. I would rather not argue semantics or get into the "who invented what" debate please. Let's just say a dissipator is a 16" barrel with the front sight post in the rifle length position. HOW TO BUILD YOUR OWN DISSIPATOR: Everything you need to know Method 1 in post #2 Everything you need to know Method 2 in post #3 The Colt Model 605A Quote:
This is the original concept as it relates to the M16, Basically just an ad hoc Carbine suited for mounted troops. It didn't last long but can be spotted in Vietnam era photos. While the idea was a sound one, it was poorly executed and later scraped. Other variants with shorter gas systems eventually replaced this design due to their reliably as a result of an increase in dwell time. The Colt 605a - Basically an M16 with the barrel cut down to 15 inches. The Concept To understand what a Dissipator is, we must first examine the purpose of such a design. 1. In close quarters or situations where individuals are mounted, a shorter barrel is desired. Shorter barrels allow freedom of movement and reduced weight while sacrificing velocity. The drop in velocity from a 20 inch barrel down to a 16 inch barrel is about 200 feet per second using the same gas system. 2. Longer sight radius allows for more precise alignment of the front and rear post. On a rifle where Iron sights are used (as is the case in an A2 rifle) the longer the sight radius the more accurate the shooter. Now one could easily overcome this compromise by using a low profile gas block and switching to a free flowing rail system that allows a rail mounted front sight to be positioned further down the barrel. This design significantly modifies the rifle with new parts that are often expensive, heavier, use proprietary mounting systems, and is not as tough as a standard A2 sight post. On a rifle with an A2 style front sight post that is also the gas block, in order to get the best of both worlds, you have to use a rifle length gas system (12inch) while using a shorter carbine length barrel. This creates a number of problems. The first being gas delivery. The AR-15 requires a reliable gas charge from a spent cartridge. As the bullet travels down the barrel, the gas pressure building up behind it is directed through a hole in the barrel, through the gas block and front sight assembly, down a gas tube and then into the bolt carrier where it cycles the bolt and carrier to complete the action. When you have a shorter barrel, the gas has less time to build pressure and travel down the tube and into the bolt carrier assembly. This is what is referred to as "dwell" time. The bullet in the barrel holds the gas in place behind it as it travels. The shorter design of 16" barrel simply does not allow for sufficient gas delivery and reliability is sacrificed. Chart showing the different velocities achieve using different gas systems and barrel lengths. Comparison of the dissipator design with the most common gas systems and barrel lengths The Bushmaster Dissipator Fast forward many years after Vietnam and welcome to the civilian AR-15. Quote:
The design uses a carbine gas system with a gas block hidden under the rifle length hardguards. Up front is a "dummy" A2 sight block that is pinned to the barrel in the rifle length position. The result is a reliably operating rifle with a longer sight radius. Bushmaster still offers their original dissipator concept upper complete with bcg for $605 (part number BURA2B 16D) Photo showing the Bushmaster Dissipator with second gas block hidden under the handguards The Other Designs The original design with a rifle length gas system and shorter barrel still exists today. Reliability is enhanced by enlarging the gas port in the barrel and thus negating the need for a second slave block that not only creates additional heat but also weight. Many SPRs use a shorter 18 inch barrel with a rifle gas system, this is considered to be the most reliable and is favored by 3 gunners and competition shooters. The dissipator in MOVIES A Marine aims his rifle during the Embassy scene in The Bourne Identity HOW TO BUILD YOUR OWN DISSIPATOR: Everything you need to know Method 1 in post #2 Everything you need to know Method 2 in post #3 Last edited by FatalKitty; 02-20-2013 at 10:04 PM.. |
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WHERE TO BUY
DPMS at JSE - http://www.jsesurplus.com/ar1516barr...eight-1-1.aspx DTI at JSE - http://www.jsesurplus.com/ar1516barr...ght-1-1-1.aspx (from what i understand, Delton makes these for DPMS or the other way around, they are the same quality, and are actually very good for the price) CMMG CHF barrels in several profiles and configs (including slave block type dissy and rifle dissy) http://cmmginc.secure-mall.com/categ...HF-Barrels-207 AIM Surplus complete upper http://www.aimsurplus.com/product.as...er&groupid=368 PSA "mock" HF mid length upper (complete) http://palmettostatearmory.com/8974.php PSA "mock" CL mid length upper (complete) http://palmettostatearmory.com/2537.php PSA also has Carbine gas system uppers but are out of stock (bottom of page) http://palmettostatearmory.com/ar-complete-uppers.php Addax Tactical has recently jumped into the fray with both a midlength/mock dissy and a full rifle length gas system http://www.addaxtactical.com/store/p...LY-57p5557.htm I will be updating this thread with new ideas, videos and photos to come HOW TO build your own Dissipator: method 1 The advantage of having a true rifle length gas system is the recoil felt. One can argue that the 5.56 does not have much recoil and would be correct. The cycling action actually causes as much recoil as the round down the chamber does. This is only exacerbated by a shorter gas system, allowing a high pressure quick pulse of gas down the tube into the BCG rocketing it back into the stock and shooter. With a rifle length gas system the felt recoil is lower. When looking at a rifle where Iron Sights are a priority - the least amount of recoil possible is desirable. You have a couple options. Either have a gunsmith cut down and rethread (if muzzle device is desired) a standard 20" barrel, or find a 16" barrel that already uses a rifle gas port. DPSM, CMMG, and others already make these barrels and they make them with enlarged gas ports for reliability. I personally have owned the DPMS version, it will cycle wolf ammo with a spikes ST2 buffer. You can trust this design both jsesurplus and midwayusa sell the DMPS version when they have them in stock (for as little as I see these rifles in the real world, the barrels sure do sell out quick) This method is simple because it only involves replacing the barrel and handguards - a job anyone with the tools can do. Parts required: 1) New barrel 2) Rifle length (12") handguards of your choice 3) Rifle length gas tube Instructions: 1) Remove carbine hand guards. 2) Secure upper. 3) Remove gas tube. 4) Remove old barrel from upper. ------------------------------ 5) Install new barrel, torque to proper specs 6) Install new gas tube 7) Install handguards RESULT: on the subject of the reliability of such a design: "It's no secret the the M16 and AR-15 have suffered a reputation that includes lack of reliability. Since the first battles of Vietnam we have heard horror stories of our soldiers dying on the battlefield with their M16s torn down in what appeared to be an attempt to clear malfunctions under a hail of AK gunfire. Soldiers decried the smaller 5.56 round is inferior to the 30-06 round of their Garands. While these flaws have long since been overcome - the reputations have stuck. Even today many preparers prefer the design of the AK-47 over the AR-15 due to it's incredible reliability - and it's hard to deny. The same is true for the shorter barrel variant. When the M16s were first chopped down to 16 inch barrels, the gas system was insufficient to cycle the bolt - a problem that has been overcome by enlarging the gas port on the barrel. But the myths still perpetuate. People on forums all over the internet will tell you how unreliable the "dissipator" design is without even understanding how it works today. Many companies still offer a true rifle length gas system on a shorter barrel with great reliability. " - FatalKitty HOW TO build your own Dissipator: method 2 Everything you need to know in post #3 below Last edited by FatalKitty; 11-19-2012 at 8:56 AM.. |
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My HBAR is a 14.5” with perm Brake. I wanted a FF dissy. Please if you do not understand a step, do a quick Google search or YouTube video search. It will most likely answer all your questions. Videos give you a great deal of knowledge since you can see for yourself what is confusing you in print form.
This combination of parts does NOT allow the front flip up BUIS to lay flat in the down position. For that to happen the Quad-rail must be 1.5” shorter or the barrel needs to be 1.5” longer. If this is going to bug you… you need other parts. Parts: 1) YHM Lightweight Rifle length Quad rail. 2) YHM HK Style Flip up front BUIS, 2piece clamp-on, no bayo lug. 3) YHM Specter 2 piece low-profile clamp-on Gas Block. 4) Barrel wrench 5) Upper block or other way of holding upper secure to torque nut. Instructions: 1) Remove carbine hand guards. 2) Secure upper. 3) Remove gas tube. 4) Remove barrel from upper. 5) Remove Delta Ring, Slip ring and Weld spring. Place them away, you will not need them. 6) Secure barrel in vise. 7) Pound out the FSB pins 8) Since the Muzzle device can’t come off, cut the FSB off. Also cut the hand guard cap off as well. This is best done by cutting 50% or slightly more through the metal, and then use pliers to twist and break the remaining metal away. This keeps the cutting wheel far away from your barrel 9) Your barrel nut should now slip off the barrel. Place it away, you will not need it. 10) Install the YHM barrel nut / Hand guard nut with the locking ring. Refer to the instructions and YouTube for a video. 11) Install the barrel to the upper and have the lock ring all the way back. 12) Install the YHM Specter Gas Block. Make sure to leave a slight gap where a hand guard cap WOULD have been. This centers the gas block hole directly over the gas port on the barrel. Even if you butt the gas block up against the barrel step up, the holes will be close enough. YHM made the gas hole in the Specter Gas Block large enough to allow this. IIRC it’s only about 1.75mm anyway. Lock this down TIGHT. Don’t bother with Loc-tite the gas block will be heating past 500 degrees which will melt the Loc-tite. You can use a perma-tex sealant, The type used for engine head gaskets. 13) Screw the FF Hand Guards all the way in…ALL THE WAY as far as possible. This allows you to install the rear clamp of the FSB, since it will be under the rail when completed. 14) Clamp the YHM front Flip up sight directly against the Muzzle device. Keep it slightly loose. 15) With the upper in a vise, hang a red string in front of the muzzle. Stand behind the upper and look into and through the barrel. The string should run straight down the front post and straight through the barrel. This is detailed in the Military Armory manual TM9-1005-319-23, Chapt 3, and page 3-42. Look at my links for the manual. Align the FSB this way and lock the screws down 16) Align the Hand guard now by unscrewing it slightly and aligning the side screws. Then use the locking ring. Here’s a crappy cell phone picture of my rifle. I will get a better shot of it this week end. Last edited by dieselpower; 10-26-2011 at 11:04 PM.. |
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Does it have to be a rifle length gas system for a 16" barrel, which I am planning to purchase, or a midlength gas will do? I have a KAC RAS M5 on hand right now and planning to build a dissy around it. I am stuck with the delta ring & end cap to secure the KAC...no floaty for this build.
@Dieselpower...do you think it would look cheesy if I use the YHM flip up BUIS to secure the front end instead of the A2 FSB? Just getting some suggestions before going further with my plan. @Thanks Fatalkitty for the topic.
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You are the architect of your fate...and sometimes the fate of others. |
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I've noticed a couple threads pop up and figured this was needed Thanks to diesel for the help and advice on this as well!
The gas block can be in carbine or midlenth position hidden under the handguards. If you're stuck with a delta ring setup, you're kind of stuck with an A2 sight block as well - the YHM and MI flip ups don't have room to mount the A2 handguard cap for standard type guards to work. I did see someone make a free floating MOE guard and I have been trying to find it and how they did it, that might be of interest to you as well. They were on calguns and the upper was for sale a few months back |
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With the advent of low profile gas blocks and rail mounted BUIS, and red dot optics, the need just isn't there for a shorter rifle with a maximized iron sight radius. -W |
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I would rather spend $50 on a new handguard and A2 sight post to convert to a "dissipator" than spend hundreds of dollars on free float rails or optics |
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-W |
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Here's some quick and dirty before and after shots of my Shrubmaster Dissipator upper.
Its a bit heavier than my M4 upper, but the Sully Stock with Carry handle and, currently, the UBR balance it out nicely. So far its been running reliably with a BCM BCG and standard carbine buffer and spring. I picked up the upper during the Obama scare from another Calgunner who said he only ran .22s threw it. Semi Kissed-Out Before Magpul Fanboyed After Last edited by Mr. Meeseeks; 10-28-2011 at 11:44 AM.. |
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I put this together with input from my son. This is what he uses on our range trips. Del-ton upper and lower, various Magpul items and a PA red dot.
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At first I didn't like the look of the MOE guards on it, but the more I see them now I love it that way, just seams to fit right That's the one! Thanks for posting that Iggy, I thought I had it in my PMs still Last edited by FatalKitty; 10-28-2011 at 1:31 PM.. |
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Terrific thread, FatalKitty. Good to have a go-to resource for all Dissipator inquiries. Would be nice if more be-all and end-all info threads like this would pop up.
--Inq
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WTB PRIMERS - SMALL PISTOL (MAGNUM OK) |
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it's something that I figured needed to be done, and I can't wait to see more photos of people's builds! |
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updated photo of the dissipator - fired here by the lovely Hannah Ray! (http://www.facebook.com/hannahraymodel) |
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That looks like my gun. Great minds think alike.
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CLEVERLY DISGUISED AS A RESPONSIBLE ADULT |
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This is a great resource!! I've consulted this thread a lot. My AR build will probably be a featureless dissipator build, either CMMG or Bushmaster. Great thread.
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REDACTED |
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Don't forget Palmentto State Armory also sell Dissy uppers with 1/7 Chrome Lined 4140 Carbine gas and 4150 mid-length gas barrels.
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Diggin this set up! May have to build one like this...
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CRPA Lifetime Member |
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Nice pix with the hottie! Is your rifle a 16" barrel, midlength or rifle length gas system?
__________________
You are the architect of your fate...and sometimes the fate of others. |
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Tangorox, my rifle uses the rifle gas system
Shot wolf all day long yestersay, stuff is crap... i could noticably feel the lesser recoil from it compared to the usual lake city stuff i shoot and it wouldnt lock back on the last round. Also had about 5 fte with it out of about 500 rounds of it. The 500 rounds of lake city i shot functioned flawlessly. I have a video of me shooting but not sure if i should post it... not wearing eye pro and im just shooting for the camera Last edited by FatalKitty; 11-03-2011 at 7:37 PM.. |
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this thread is making me miss my old Dissi witha A1 stock.....
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"We are not human beings having a spiritual experience; we are spiritual beings having a human experience." - Pierre Teilhard de Chardin. |
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I've got a 20" rifle length port and barrel I'm going to have cut to 16". The question is, what is the size of the port on reliable rifle length dissipators? Is there enough dwell time to cycle the rifle reliably? Not that its of any consequence for CA rifles, but I am curious how a rifle length ported barrel at 16" would function in full auto? Would it be reliable as well?
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the DPMS barrel I have cycles just fine under rapid fire |
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looking into building a dissy and had a few questions.
right now im debating between rifle or midlength. the one issue i have with rifle length is whether or not i will run into problems with over/under gassing with certain ammo. i know fatalkitty has already posted he had issues with wolf and i'm a little iffy if any other ammo might turn up the same. also which buffer combo should one run with a dissy? is it different if i change the gas system from rifle to mid to carb? |
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You won't have to worry about overgassed with a dissy... if anything you would run into something that is undergassed. But really, there aren't many ARs out there that will reliably cycle wolf ammo - in fact my dissy does it better than my carbine. it runs all other steel ammo just fine... but wolf is crap and my guns hate it.
The buffer is dependant on your stock, not the gas system. a "rifle buffer" is for the A2 stock that is a fixed rifle stock... the "carbine buffer" is for a carbine stock like a collapsible stock the cool thing about about using a mock dissy is that you can have free float handguards and a lighter setup (can only find HBAR or mid dissy barrels right now) and it seams to be the way everyone does it. I went with the rifle gas systems for a couple of reasons: 1. not many had that set up and I wanted it 2. heavy barrel keeps the muzzle climb down 3. smoother operating gas system - as this gun was going to be iron sights almost exclusively I wanted to keep muzzle jump and recoil down as much as possible 4. I wanted to build it myself, something I wasn't able to do for my first AR with a dual gas block setup (this requires modifying the barrel so that the A2 front post can be pinned in place Last edited by FatalKitty; 11-14-2011 at 5:30 PM.. |
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This thread is making me want one of the JSE DPMS or PSA dissipators for a play-around/go-fast featureless rifle...
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