SIXXgun Posted January 4, 2007 Share Posted January 4, 2007 I'm looking to buy my first AR and don't really understand the gas system lengths, but from what I'm reading I would be better off with a longer one. However, I don't really want to carry a long barreled carbine. I would mostly be shooting short range up to 150 yds or so in standard division. What can you tell me about the dissipator style setups? Barrel length and Gas system style is what I'm looking for. JM Link to comment Share on other sites More sharing options...
George Posted January 4, 2007 Share Posted January 4, 2007 Get the full length gas system so it will shoot smooth, then go with the shortest bbl that will work with a FL gas system which is I believe 17.5er. Not that differnt' than a 16er and will work much better while being just as handy. You DO NOT want a 10-12, or even a 14"er with dissipator for this sport, trust me on this one ;-) Link to comment Share on other sites More sharing options...
SIXXgun Posted January 4, 2007 Author Share Posted January 4, 2007 I thought the dissipator rifles were 16" or 20" barrels. I guess there are lots of different lengths. JM Does a 16" dissipator run on a short gas system? JM Link to comment Share on other sites More sharing options...
StealthyBlagga Posted January 4, 2007 Share Posted January 4, 2007 Never never never get a non-NFA AR15 built with a carbine gas system. Too much gas results in excessive port erosion and over-violent cycling. If you must have a 16" barrel, the most reliable gas system will be a midlength (half way between carbine and rifle gas systems). If you are using optics, an off-the-shelf flattop midlength will work just fine. If you are using irons, you want to maximize your sight radius; get a midlength gas system, cut off the top of the FSB and install a second "dummy" gas block/front sight just behind the muzzle brake (if you like the Dissipator look, you can install a rifle length FF tue over the chopped gas block). If you go for something in the 17.5-18" range, either the midlength or rifle gas systems are workable options. Above 18", go for the rifle length gas system. Personally, I prefer a longer and heavier gun for IPSC/3-gun (I shoot a 20" HB in competition, but thats just me). If it helps, here are pics of my 16" midlength carbine and my 20" rifle: I thought the dissipator rifles were 16" or 20" barrels. I guess there are lots of different lengths. No. The Dissipator was originally a Bushmaster creation comprising a 16" barrel with a carbine length gas system with a rifle length sight radius. The gas block is hidden under the handguards. Does a 16" dissipator run on a short gas system? The Bushmaster Dissipator does indeed use the (undesirable) carbine gas system. Other manufacturers offer Dissipator copy-cats; some have carbine gas systems (too violent) ohters have rifle gas systems (too unreliable). Like I said - go with a middy. Link to comment Share on other sites More sharing options...
SIXXgun Posted January 4, 2007 Author Share Posted January 4, 2007 What's a non-NFA AR15? I know jack about AR15's - always shot bolt actions. I like the 16" flat top M4 style rifles, but everybody dogs the short gas system. So I get to thinking maybe I should look for a longer gas system to begin with instead of dropping a lot of cash on something I wouldn't be happy with in the long run. I'm assuming: a gas tube runs the length of the barrel from the chamber to the gas block. When this tube fills during ignition, the gas pressure builds against the block and then blows backwards, cycling the action. If I've got it pretty close, then I guess it would not be too hard to put on a little longer gas tube, move the gas block forward, and now I've got a mid length gas system. Am I close? Please educate me since I don't have any experience with this. A 16" dissipator has a longer handguard, but I'm also assuming it has a short gas sytem - a working gas block covered up by the handguard and then a front sight which is attached to a dummy gas block. Could the gas block under there be removed and the gas tube attached to the block in front of the handguard (right behind the muzzle break) to make a mid-length gas system? Is this hard to do, or am I totally lost?? Please keep the replies coming, and bear with me while I learn about all of this stuff. Thanks guys. JM Link to comment Share on other sites More sharing options...
cking Posted January 4, 2007 Share Posted January 4, 2007 Since you mention that 150 yards. I recommend a mid-length gas system setup on a 16/18 barrel. Now barrel weight and balance is kinda of a personal preference. I started with a Colt 16 Hbar barrel short gas system and then did a 18" Hbar with mid length. Then I tried a friends old style pencil barrel 20 inch gun. Found it swung better. So for my third try I went with 18 mid weight barrel .75 diameter the whole length along with a lefty stag upper. For me third time was perfect. Plus I like building them and building AR's is really easy. If you want to try something special I recommend Frank White at compass lake engineering in about 4 weeks he will build you whatever you want with all the popular blanks available. Have fun it is just money Link to comment Share on other sites More sharing options...
StealthyBlagga Posted January 4, 2007 Share Posted January 4, 2007 (edited) What's a non-NFA AR15? Sorry for the jargon. NFA means "National Firearms Act", and generally refers to gun required to be registered with the Federal Government; in this case, I am referring to Short Barrel Rifles (SBRs) or Machineguns (MGs). IMHO, the only reason to have a short/carbine gas system is because your barrel is below 16" and thus will not work with a midlength or rifle gas system. A "non-NFA AR15" is a rifle with a barrel at least 16" long (i.e. the kind of gun you can buy from any local gunshop just by completing a 4473 form). For the purposes of serious 3-gun competition, we always use non-NFA rifles, and so the short/carbine gas system should be avoided. I like the 16" flat top M4 style rifles, but everybody dogs the short gas system. So I get to thinking maybe I should look for a longer gas system to begin with instead of dropping a lot of cash on something I wouldn't be happy with in the long run. Forget the M4 (even though it looks cool). Get a midlength instead... same functionality, but much more reliable, will last longer and shoots smoother. Any AR15, including the midlength, can be had with a flat top, flash hider, M4 stock etc. etc. I'm assuming: a gas tube runs the length of the barrel from the chamber to the gas block. When this tube fills during ignition, the gas pressure builds against the block and then blows backwards, cycling the action. If I've got it pretty close, then I guess it would not be too hard to put on a little longer gas tube, move the gas block forward, and now I've got a mid length gas system. Am I close? Please educate me since I don't have any experience with this. Not quite. The gas bleeds from the barrel via a hole drilled in the barrel under the gas block (this hole is known as the "gas port"). This gas is fed back (via the gas tube) to the bolt carrier, pushing it back, unlocking the bolt and cycling the action. Gas is blown directly into the bolt carrier assembly and upper receiver - this method of operation is unique to the AR series of rifles, and is called "direct gas impingement". When you understand that a hole is drilled in the barrel, then you can see why it is not trivial to relocate the gas block. Instead, you need to choose your gas system (short/carbine, midlength or rifle) and then acquire the appropriate barrel already drilled with the gas port of the right size and location. A 16" dissipator has a longer handguard, but I'm also assuming it has a short gas sytem - a working gas block covered up by the handguard and then a front sight which is attached to a dummy gas block. Could the gas block under there be removed and the gas tube attached to the block in front of the handguard (right behind the muzzle break) to make a mid-length gas system? Is this hard to do, or am I totally lost?? Yes, you could MAYBE do this, depending on how the rifle was originally built, but why would you want to ? Just buy the exact configuration you want from the get go... no need to relocate or replace any parts. Edited January 4, 2007 by StealthyBlagga Link to comment Share on other sites More sharing options...
chuckbradley Posted January 4, 2007 Share Posted January 4, 2007 I use a dissapator. I love it. Short barrel for fast swinging in close target situations but still a full sight radius. This is important since I shoot limited. I use the globe sights. Yes the gas block is under the handguard. I found it to be the perfect fit for shooting limited rifle, at least for me. Link to comment Share on other sites More sharing options...
George Posted January 4, 2007 Share Posted January 4, 2007 The "Downside" to using a short gas system (what I think of as Dissipator style rifles) is the too early timing and additional amplitude of their gas pulse and what it does to cases and the extractor. The case is blown against the chamber walls by pressure and this creates friction that helps hold the case in place. If you try to extract the case before it springs back down in size after the bullet clears the muzzle and the pressure spike has subsided, you are pulling against this stiction. Until this point is reached, attempting to extract the case will actually rip the case out of the chamber against this extra friction instead of just sliding it out. This creates scratch marks on the cases and extra wear and tear on the chamber walls. It also beats the heck outa' the extractor and this part fails early on short gas systems. If the chamber is not kept scrupulously clean and it is rough in the first place, this can create lots of extra extraction force resistance to be applied to the bolt and in some cases the extractor pulls over the case rim because the bolt ain't stopping. Bad juju while shooting. Not a huge issue, but one that does crop up ocassionally when issues stack up. Get the long gas system if at all possible, if not at least get a middie. The timing is not so far off with a middie gas pulse. A shortie's pulse timing however is just plain wrong and Eugene Stoner is turning over in his grave on this one ;-) Link to comment Share on other sites More sharing options...
Bear1142 Posted January 4, 2007 Share Posted January 4, 2007 Alright, enough of the drama. While all of the arguments against a short gas system may be "theoretically" accurate, it does not reflect "reality." Those who criticize the short gas system design would have you believe a short gas system AR is completely unreliable and the work of the devil. While it may be true that the design is of devilish origin, the short gas system AR has been around for at least 35 years. Manufacturers and designers have had plenty of time to work out the timing, pressure, and port size issues. I have personally had and have knowledge of dozens and dozens of M4 style, short gas system AR's that have been just as reliable as the old standard 20" system. In the vacuum of a laboratory, all of the short gas system concerns may be valid, but don't believe the hype. I used a Dissipator style setup for the first two years of 3-gun competition and did quite well with it. A good compensator and good load selection will flatten out the increased pulse of a short gas system to the point of being practically equal to a rifle length gas system. If I were building a rifle from the ground up, yes, I would go with an 18" barrel with a rifle length gas system. If I had an M4 sitting around and I could not afford to build a custom rifle, I would put on a good compensator, start shooting, and not worry that my short gas system rifle won't work, because it will. For a bunch of people who always claim that it's the singer and not the song, it seems to only be true if you only sing the approved song. Not flaming anyone, just trying to balance the discussion points. Erik Link to comment Share on other sites More sharing options...
TommiF Posted January 4, 2007 Share Posted January 4, 2007 go with the shortest bbl that will work with a FL gas system which is I believe 17.5er. George, Again I have to correct You I changed one of my older barrels from carbine to rifle. Barrel is 16 inch long. There is about 71mm from gashole to crown. Hole is drilled to 3mm (about 0,12 inch) and gun works fine, also with guite soft training ammo. With NATO spec ammo, there is absolutely no problem. Tommi Link to comment Share on other sites More sharing options...
kurtm Posted January 4, 2007 Share Posted January 4, 2007 Break out the Champagne! Bear and I finally agree! Yes the recoil impulse is a wee bit more, but I have won matches with both types of gas systems, A good comp really helps A LOT! I have been told that the "lightened" carriers won't work in short systems also, Guess what, don't tell Mike Pinto, he has a couplke of thousand rounds through a M-4 style Rock River with the light carrier no problem. Now I do like the 20" barrels in general but it has more to do with ballistics than it does recoil. Mikes rifle is very soft and flat with a Rolling Thunder comp, and lightened carrier, and I can't tell much difference after the beep! KURT Link to comment Share on other sites More sharing options...
Bear1142 Posted January 4, 2007 Share Posted January 4, 2007 Break out the Champagne! Bear and I finally agree! WOO-HOO! How about some dark brew instead? Erik Link to comment Share on other sites More sharing options...
StealthyBlagga Posted January 4, 2007 Share Posted January 4, 2007 I don't think anyone is saying that the short gas systems never work. We are saying that they are more likely to have reliability and robustness problems than a middy or rifle length gas system. Like many here, over the years I have owned (and won local 3-gun matches) with short gas system rifles... a Dissipator and an M4gery. However, both guns showed more violent extraction and suffered from maddeningly intermittent ejection issues. The M4gery suffered port erosion so bad that I had to install an adjustable gas tube and crank it to the fully-closed position to minimize the excessive bolt speed. I will never use a short gas system on a 16" AR15 again. Remember, this poor guy knows nothing about ARs, and has the good fortune to be starting with a clean sheet of paper... lets not burden him with the distraction of a potentially unreliable rifle when he should be focussing on the fundamentals of practical shooting. Counsel for the midlength rests his case . Link to comment Share on other sites More sharing options...
George Posted January 4, 2007 Share Posted January 4, 2007 Ain't sayin' they (shorty gasser's that is) don't work, they are just outa' the "nice" range in the way they handle the cases and the extractor issue is valid in that it WILL wear out earlier. They will run fine of course, the amount of gas available insures that. The extractor will suffer more, the cases it ejects will not be as prized for reloading and the cycling impulse (NOT the recoil impulse) will be more noticeable. Reliability isn't the issue, it's the things a shortie does that are not nice. It's like running open pipes on a two-stroke engine timed for expansion chambers. Without the backpressure the chamber provides, port timing is wrong and it makes extra noise (equate this with extra cycling pulse). None of these things make it harder to ride fast, just less pleasant and harder on the parts in the long run. These are real world facts, not "lab stuff". No flamin' here either, just plain facts! If a short gas system rifle was all I had, it's what I would shoot and it would not effect my score one iota, just the way I feel about the gun and the way I maintain it. If I were putting money down a new barrel though, it would have to be a mid, or long gas system NOT a shorty. BTW, Tommi, I didn't think it couldn't BE done, I just thought it hadn't been done ;-) Link to comment Share on other sites More sharing options...
uscbigdawg Posted January 4, 2007 Share Posted January 4, 2007 Erik & Kurt beat me to it. My 16" carbine length gas'ed rifle runs like a champ. While I think a bad rifle is just a bad rifle, I attribute the reliability of this rifle solely with Benny Hill's TLC. In shooting it in CQ shots, it's still a little long for my personal preference, but much better than even my cherished 18" JP/Voigt rifle. The recoil/gas impulse is noticably a little more harsh, but with the Rolling Thunder comp, at rapid long shots, it was fast and flat and I was running a full weight carrier. On the next rifle, it'll be a 16" ML gas with JP-SS-LMOS and Rolling Thunder comp. I think this'll be the heat. Bottom line is that 16" rifles are viable options, especially in Open Division 'cause sight radius is a non-issue. If I was shooting Limited, 18" would be the shortest barrel I'd shoot. 20" would be the ideal with an extra gas block just aft of the comp with a front sight mounted on it. And there'd be a rifle length gas system in there just to keep everything smooooooth. Rich Link to comment Share on other sites More sharing options...
Bear1142 Posted January 4, 2007 Share Posted January 4, 2007 and the extractor issue is valid in that it WILL wear out earlier. I agree...that's why I replace my M4 extractors every 10,000 rounds whether they need it or not! Erik Link to comment Share on other sites More sharing options...
uscbigdawg Posted January 4, 2007 Share Posted January 4, 2007 and the extractor issue is valid in that it WILL wear out earlier. I agree...that's why I replace my M4 extractors every 10,000 rounds whether they need it or not! Erik Ditto. We (Competition Shooters) beat the piss out of our guns and parts wear out. I replace the gas rings, bolt cam pin, firing pin, firing pin retaining pin, extractor and extractor spring every 10k and most times it doesn't need it. Why? Cheap insurance. I also keep extra parts in my range bag. One thing that George eluded to I tend to do as well, for my own OCD in that I Boresnake my rifle between stages. Rich Link to comment Share on other sites More sharing options...
SIXXgun Posted January 4, 2007 Author Share Posted January 4, 2007 Thanks for the help. I think I will just stick to the basics with a stock rifle for now since I've never even shot in a carbine match. I appreciate all the good information and posts which will help me decide what model I end up buying. Now I can come up with a list of features to look for and questions to ask. Thanks again. JM Link to comment Share on other sites More sharing options...
Kimberkid Posted January 7, 2007 Share Posted January 7, 2007 I am in about the exact same situation right now, in that I will be ordering my first upper, and dont know shit about AR's. Is it possible to put a full length gas system in a 16 in. Dissapator top end? It looks like no, from the above comments, but a friend of mine says yes. If I understanding my friend he is telling me since the front sight is the end of the gas system, and the Dissapator has the same sight radius as a standard rifle, it would accomadate a full length gas tube. I am hoping to just order a complete top end, but I dont wanna order it and then realize it isnt what I was looking for. I am intending to order this top end, put on a scope and shoot Tactical division. Is this not going to work? Am I likely to make power factor with factory ammo? I am getting different answers from several sources on all these questions, and the variety of options is making my head spin. Link to comment Share on other sites More sharing options...
uscbigdawg Posted January 7, 2007 Share Posted January 7, 2007 You can have a 16" barrel and rifle length gas system, but it'll probably have to be cut down to that length by a gunsmith (see TommiF's post/picture). As for making PF with factory ammo, always run it over a chrono but through my 16" w/ Ultramax Remanufactured 55gr. FMJ's I made it with room to spare and it was coooooold when we shot over the chrono. I'm quite certain a good worked up 62-69gr. load will do the trick nicely. A good source for ammo that I've found in doing some BRIEF testing was Georgia Arms. They have Hornady 68gr. supposedly hitting around 2800fps. I did some plinkin' with their loads and zeroed my JPoint to it (didn't get around to zeroing the ACOG) and it shot great! Obviously much better BC than the 55's. Rich Link to comment Share on other sites More sharing options...
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