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Posted: 2/10/2009 7:04:20 AM EDT
[#1]
You can go to 18" but that length is better suited to rifle gas.
14.5" carbine, 16" midlength, 18" rifle. |
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Posted: 2/10/2009 7:58:25 AM EDT
[#2]
You're asking the wrong question. If you're going to the trouble and expense of having a barrel custom turned it's exceedingly foolish to let the length of a FF tube you happen to have on hand dictate what length and gas system you use. Step 1. Define your "mission". Step 2. Decide what barrel length you need to fulfill the aforesaid mission. Step 3. Use the longest gas system reasonable.
If it doesn't happen to be a mid length gas system sell the FF tube and buy whatever it is you need. FWIW I would never put a mid length gas system on anything longer than 17". 18" barrels are best with rifle gas, 16" down to 14.5" are beds with mid gas, and shorter than gets car or pistol. Regardless of what the .mil thinks 14.5" barrels run better with mid gas. In your case, if you're completely wedded to the middy idea, I would go with a 16" or 14.5" barrel. "Getting the most" out of your rifle doesn't necessarily mean only the most velocity. Other factors such as maneuverability also come into play. |
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Posted: 2/10/2009 8:30:38 AM EDT
[#3]
Quoted:
You're asking the wrong question. If you're going to the trouble and expense of having a barrel custom turned it's exceedingly foolish to let the length of a FF tube you happen to have on hand dictate what length and gas system you use. Step 1. Define your "mission". Step 2. Decide what barrel length you need to fulfill the aforesaid mission. Step 3. Use the longest gas system reasonable. If it doesn't happen to be a mid length gas system sell the FF tube and buy whatever it is you need. FWIW I would never put a mid length gas system on anything longer than 17". 18" barrels are best with rifle gas, 16" down to 14.5" are beds with mid gas, and shorter than gets car or pistol. Regardless of what the .mil thinks 14.5" barrels run better with mid gas. In your case, if you're completely wedded to the middy idea, I would go with a 16" or 14.5" barrel. "Getting the most" out of your rifle doesn't necessarily mean only the most velocity. Other factors such as maneuverability also come into play. I wouldn't go longer than 16" if you do midlength. Longer than 16" I would go with an intermediate or rifle length gas system. |
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Posted: 2/10/2009 8:55:17 AM EDT
[#4]
Quoted:
You're asking the wrong question. If you're going to the trouble and expense of having a barrel custom turned it's exceedingly foolish to let the length of a FF tube you happen to have on hand dictate what length and gas system you use. Step 1. Define your "mission". Step 2. Decide what barrel length you need to fulfill the aforesaid mission. Step 3. Use the longest gas system reasonable. If it doesn't happen to be a mid length gas system sell the FF tube and buy whatever it is you need. FWIW I would never put a mid length gas system on anything longer than 17". 18" barrels are best with rifle gas, 16" down to 14.5" are beds with mid gas, and shorter than gets car or pistol. Regardless of what the .mil thinks 14.5" barrels run better with mid gas. In your case, if you're completely wedded to the middy idea, I would go with a 16" or 14.5" barrel. "Getting the most" out of your rifle doesn't necessarily mean only the most velocity. Other factors such as maneuverability also come into play. I tend to agree that (in general) a mid gas system is probably a better option for a 14.5" bbl. - vs. a carbine gas system ... That said, however, IMHO neither of those to two is actually the optimal solution for a 14.5" bbl. |
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Posted: 2/10/2009 8:56:51 AM EDT
[#5]
I'm not going to be clearing rooms or anything tacticool like that...
I will probably put a 3-9x scope on it and mostly plink, 100-400yd. Preferable barrel length between 16 and 22 inches. I already have stuff for a middy, so instead of trying to find more parts and spend more money, i'm using what I have. I'd like to get the most velocity I can, because it's nice to have. Why not go beyond 16" with a middy? |
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Posted: 2/10/2009 8:57:53 AM EDT
[#6]
Quoted:
That said, however, IMHO neither of those to two is actually the optimal solution for a 14.5" bbl. What's the optimal solution for a 14.5 in your opinion? |
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Posted: 2/10/2009 9:26:36 AM EDT
[#7]
14 feet
16" then go to rifle length gas |
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Posted: 2/10/2009 9:33:57 AM EDT
[#8]
Quoted:
Quoted:
That said, however, IMHO neither of those to two is actually the optimal solution for a 14.5" bbl. What's the optimal solution for a 14.5 in your opinion? Given the numerous problems which were initially experienced with the 14.5" bbl. + carbine gas system combination, it seems as though it could have or should have been something in between the carbine and the mid ... Much like the development of the intermediate length system for the 18" bbl., which is essentially halfway between a 16" mid and a 20" rifle. But as far as I can tell, there was never much (if any) "effort" put into finding the sweet spot for a gas system on a bbl. length that ended up being close to the half-way point between an 11.5" bbl. and a 16" bbl. That said, the 16" midlength - in common use - is a relatively recent occurrence, and "hindsight is always 20/20". And again, my thoughts on gas system lengths for 14.5" bbls. are all IMHO. |
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Posted: 2/10/2009 11:02:38 AM EDT
[#9]
Actually, I think that mid gas on a 14.5 is pretty close to optimal.
Here's the greatly simplified, gas system theory 100: You want the bolt to remain locked for as long as possible to allow the brass to contract. It takes a certain function of (Gas Pressure x Port Size x Pulse Duration / Time After Ignition) to reliably operate the weapon over a range of conditions. Each of these variables has an optimum range. Overcoming a deficiency in one variable, say lack of duration as in a 16" rifle gas gun, requires another variable to be increased outside of it's optimum range. In this case the gas port is opened up to nearly half the bore diameter. In the case of putting a long barrel on a mid gas gun you're increasing both Gp and D while reducing T. Obviously to maintain the balance you have to reduce Ps. That's it in a nut shell. 
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Posted: 2/10/2009 11:22:17 AM EDT
[#10]
Quoted:
Actually, I think that mid gas on a 14.5 is pretty close to optimal. Here's the greatly simplified, gas system theory 100: You want the bolt to remain locked for as long as possible to allow the brass to contract. It takes a certain function of (Gas Pressure x Port Size x Pulse Duration / Time After Ignition) to reliably operate the weapon over a range of conditions. Each of these variables has an optimum range. Overcoming a deficiency in one variable, say lack of duration as in a 16" rifle gas gun, requires another variable to be increased outside of it's optimum range. In this case the gas port is opened up to nearly half the bore diameter. In the case of putting a long barrel on a mid gas gun you're increasing both Gp and D while reducing T. Obviously to maintain the balance you have to reduce Ps. That's it in a nut shell. I would agree... optimal for a 14.5 might be a little shorter than midlength, but it's probably not far off. IMO, the midlength gas system was originally conceived using faulty criteria - maintain the same length of barrel past the gas port at a 20" barrel in a 16" barrel - rather than on a functional analysis to optimize the gas system function, although it IS an obvious and dramatic improvement over carbine length gas in that barrel length. My SWAG is that KAC actually DID some functional analysis to come up with their gas system length for the SR15 E3 rifles, as it is somewhere in the range of where I would expect the optimum length to be - IMO, as barrel length goes down, gas system length should go down by a slower proportion to compensate for higher pressure in the gas system. As a rough estimate, compare a carbine gas 11.5" barrel to a 20" rifle gas barrel and extrapolate the proportions for lengths in between. |
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Posted: 2/10/2009 11:50:45 AM EDT
[#11]
Quoted:
IMO, the midlength gas system was originally conceived using faulty criteria - maintain the same length of barrel past the gas port at a 20" barrel in a 16" barrel You sure about that? I had always heard that that was the design criteria for the carbine gas system. Keeping the same amount of barrel after the gas block (bayonet lug) on a 14.5" barrel. And that the middy was a redesign to overcome the over gassing of 16" civilian guns. I haven't seen any real good evidence either way though, just hearsay. |
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Posted: 2/10/2009 11:58:19 AM EDT
[#12]
The answer is also caliber dependant.
For example a midlength is perfectly fine (and suggested) for an 18" 6.8 SPC. |
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Posted: 2/10/2009 12:03:47 PM EDT
[#13]
KAC midlength
Frank White has our dimensions, it makes for an even flatter shooting 16" gun. Given you already have the middy tubes, I would suggest the Armalite midlenght for a 16" gun based on what you seem to want. |
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Posted: 2/10/2009 12:03:53 PM EDT
[#14]
Quoted:
Quoted:
Actually, I think that mid gas on a 14.5 is pretty close to optimal. Here's the greatly simplified, gas system theory 100: You want the bolt to remain locked for as long as possible to allow the brass to contract. It takes a certain function of (Gas Pressure x Port Size x Pulse Duration / Time After Ignition) to reliably operate the weapon over a range of conditions. Each of these variables has an optimum range. Overcoming a deficiency in one variable, say lack of duration as in a 16" rifle gas gun, requires another variable to be increased outside of it's optimum range. In this case the gas port is opened up to nearly half the bore diameter. In the case of putting a long barrel on a mid gas gun you're increasing both Gp and D while reducing T. Obviously to maintain the balance you have to reduce Ps. That's it in a nut shell.
I would agree... optimal for a 14.5 might be a little shorter than midlength, but it's probably not far off. IMO, the midlength gas system was originally conceived using faulty criteria - maintain the same length of barrel past the gas port at a 20" barrel in a 16" barrel - rather than on a functional analysis to optimize the gas system function, although it IS an obvious and dramatic improvement over carbine length gas in that barrel length. My SWAG is that KAC actually DID some functional analysis to come up with their gas system length for the SR15 E3 rifles, as it is somewhere in the range of where I would expect the optimum length to be - IMO, as barrel length goes down, gas system length should go down by a slower proportion to compensate for higher pressure in the gas system. As a rough estimate, compare a carbine gas 11.5" barrel to a 20" rifle gas barrel and extrapolate the proportions for lengths in between. Interestingly, and IIRC, Steve at ADCO also indicated in a post (some time ago) that technically a 16" bbl. is not actually the exact, optimal bbl. length for the midlength gas system either ... but that it is still very close. And as you mentioned, it's clearly a whole lot better than a 16" carbine system. |
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Posted: 2/10/2009 12:10:32 PM EDT
[#15]
Quoted:
KAC midlength
Frank White has our dimensions, it makes for an even flatter shooting 16" gun. Does he have or are gas tubes available for this? I've tried several times to get actual info on the KAC length system as far as it's dimensions and how it came to be (other than my guessing). I would like to get a couple barrels made with this length and had already talked to Frank but didn't mention that specifically. Quoted:
Quoted:
IMO, the midlength gas system was originally conceived using faulty criteria - maintain the same length of barrel past the gas port at a 20" barrel in a 16" barrel You sure about that? I had always heard that that was the design criteria for the carbine gas system. Keeping the same amount of barrel after the gas block (bayonet lug) on a 14.5" barrel. And that the middy was a redesign to overcome the over gassing of 16" civilian guns. I haven't seen any real good evidence either way though, just hearsay. The 14.5" barrel came about via the opposite path - taking the existing carbine length gas system from the Vietnam era XM177 types and extending the barrel to get the same barrel length in front of the gas port as on the 20" rifles. |
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Posted: 2/10/2009 12:14:29 PM EDT
[#16]
Quoted:
KAC midlength
Frank White has our dimensions, it makes for an even flatter shooting 16" gun. Given you already have the middy tubes, I would suggest the Armalite midlenght for a 16" gun based on what you seem to want. With the understanding that KAC would be your first choice ... I'm curious to know what it is about the Armalites that you like over (any of) the other manufacturers who are making middys ? Anything specific about their product that you really like - or is it something as simple as a "personal preference" kind of thing ? |
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Posted: 2/10/2009 12:37:01 PM EDT
[#17]
Quoted:
I'm curious to know what it is about the Armalites that you like over (any of) the other manufacturers who are making middys ? Armalite/Eagle Arms developed and brought what we now know as "midlength" to market. He's saying "Armalite midlength" to distinguish from "KAC midlength" as a description of the configuration, not for a brand suggestion. What we know as "midlength" in the commercial marketplace is "Armalite midlength". |
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Posted: 2/10/2009 12:44:16 PM EDT
[#18]
Quoted:
Quoted:
I'm curious to know what it is about the Armalites that you like over (any of) the other manufacturers who are making middys ? Armalite/Eagle Arms developed and brought what we now know as "midlength" to market. He's saying "Armalite midlength" to distinguish from "KAC midlength" as a description of the configuration, not for a brand suggestion. What we know as "midlength" in the commercial marketplace is "Armalite midlength". roger that. |
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Posted: 2/10/2009 12:49:11 PM EDT
[#19]
Ahhh ..... got it. Thanks. |
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Posted: 8/6/2009 1:03:34 PM EDT
[#20]
Bump to save from archive...
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Posted: 10/20/2009 8:25:15 PM EDT
[#21]
Quoted:
Quoted:
Actually, I think that mid gas on a 14.5 is pretty close to optimal... <snip> I would agree... optimal for a 14.5 might be a little shorter than midlength, but it's probably not far off. IMO, the midlength gas system was originally conceived using faulty criteria - maintain the same length of barrel past the gas port at a 20" barrel in a 16" barrel - rather than on a functional analysis to optimize the gas system function, although it IS an obvious and dramatic improvement over carbine length gas in that barrel length. My SWAG is that KAC actually DID some functional analysis to come up with their gas system length for the SR15 E3 rifles, as it is somewhere in the range of where I would expect the optimum length to be - IMO, as barrel length goes down, gas system length should go down by a slower proportion to compensate for higher pressure in the gas system. As a rough estimate, compare a carbine gas 11.5" barrel to a 20" rifle gas barrel and extrapolate the proportions for lengths in between. Gamma762, Here's some additional info (thought you might find it interesting) for your review, study, "FYI" ... from THIS thread: Quoted:
Can anyone explain the "rationale" or reasoning behind the unique gas system length they have developed with this weapon ? They must (obviously) consider their system to be superior to any of the Armalite-type midlength gas systems, so I'm curious to know (with respect to 16" bbls.) why it's better ... Again, just curious. Quoted:
Quoted:
<snip> THE KAC PROPRIETARY MID-LENGTH GAS SYSTEM, AT 1.3" LONGER THAN THE ARMALITE "INDUSTRY STANDARD" MID-LENGTH GAS SYSTEM, YIELDS A NOTICEABLY SMOOTHER GAS PULSE WITH A 16" BARREL. YOU GET ALMOST THE SAME SENSATION SHOOTING WITH THE KAC SR15 E3 AS YOU DO SHOOTING A COMPENSATED MID-LENGTH 16" GUN, BUT WITHOUT ALL OF THE NOISE. IT REMINDS ME OF OUR PROPRIETARY 17" COMPETITION BARREL USING A RIFLE LENGTH GAS SYSTEM. THE INTERESTING PART OF THE STORY IS HOW THE KAC MID-LENGTH GAS SYSTEM GOT TO BE 1.3" LONGER IN THE FIRST PLACE. Quoted:
THE GAS TUBE IN QUESTION WAS DEVELOPED BACK IN THE EARLY DAYS AT KNIGHT'S, WHEN EUGENE STONER WORKED THERE, SPECIFICALLY FOR THE KAC SR-25K 16" CARBINE. WHEN A NEED FOR A MID-LENGTH GAS SYSTEM CAME UP FOR A 16" 5.56MM, IT WAS THEIR NATURAL CHOICE, SINCE IT WAS ALREADY IN THEIR SYSTEM. Evidently, the KAC SR-25K 16" Carbine was chambered for a 7.62 round ... |
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Posted: 10/20/2009 9:16:42 PM EDT
[#22]
Quoted:
Gamma762, Here's some additional info (thought you might find it interesting) for your review, study, "FYI" ... from THIS thread: Vewwy intewesting.... As an aside I've been curious if it's the same length as the really early experimental rifle pictured here, the first known "midlength".... 
I did look for that particular rifle during the KAC Museum tour but it didn't appear to be on display  (Bottom rifle on the left side)
I'd be curious to look at the wear pattern on a SR15-E3 bolt that's had a couple thousand rounds through it. |
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Posted: 10/21/2009 1:33:15 AM EDT
[#23]
I have an 18" mid length that runs flawlessly. I went with the middy after seeing several rifle length gas systems on an 18" not cycle reliably in very cold weather. YMMV
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Posted: 10/21/2009 4:56:22 AM EDT
[#24]
I have an 18" Sabre middy barrel. It works just fine. Hot weather, cold weather, doesn't matter.
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Posted: 10/21/2009 7:11:22 AM EDT
[#25]
Quoted:
The answer is also caliber dependant. For example a midlength is perfectly fine (and suggested) for an 18" 6.8 SPC. Thank you for saying that. I started getting worried about my next build plans. Thought I would have to reconfigure everything. 
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Posted: 10/21/2009 7:28:30 AM EDT
[#26]
Quoted: You're asking the wrong question. If you're going to the trouble and expense of having a barrel custom turned it's exceedingly foolish to let the length of a FF tube you happen to have on hand dictate what length and gas system you use. Step 1. Define your "mission". Step 2. Decide what barrel length you need to fulfill the aforesaid mission. Step 3. Use the longest gas system reasonable. If it doesn't happen to be a mid length gas system sell the FF tube and buy whatever it is you need. FWIW I would never put a mid length gas system on anything longer than 17". 18" barrels are best with rifle gas, 16" down to 14.5" are beds with mid gas, and shorter than gets car or pistol. Regardless of what the .mil thinks 14.5" barrels run better with mid gas. In your case, if you're completely wedded to the middy idea, I would go with a 16" or 14.5" barrel. "Getting the most" out of your rifle doesn't necessarily mean only the most velocity. Other factors such as maneuverability also come into play. Well said, but you forgot to bring caliber in with it. For calibers other than 5.56 the carbine/mid-length/rifle gas system rules don't apply. For example with a 6.8 SPC 18" barrels run best on mid-length gas systems, and will occasionally choke on rifle length gas systems. |
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Posted: 10/22/2009 5:10:03 AM EDT
[#27]
Just to stir the pot a little, has anyone seen that Sabre is offering a 20" mid gas barrel? LINK
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Posted: 10/22/2009 8:01:27 AM EDT
[#28]
Interesting thread with a lot of good info. Out of curiosity what is the shortest one should go with a midlength (I heard a few in here say 14.5)? Why not shorter? What issues would one run into
with say a 12.5 inch barrel w/ midlength? |
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Posted: 10/22/2009 9:29:06 AM EDT
[#29]
Quoted:
Interesting thread with a lot of good info. Out of curiosity what is the shortest one should go with a midlength (I heard a few in here say 14.5)? Why not shorter? What issues would one run into with say a 12.5 inch barrel w/ midlength? Insufficient energy for reliable operation. |
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Posted: 10/22/2009 10:52:25 AM EDT
[#30]
for 5.56, 18" would be fine, people do it... its overgassed, but wont hurt anything.
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Posted: 1/22/2010 1:02:41 PM EDT
[#31]
Quoted:
Quoted:
You're asking the wrong question. If you're going to the trouble and expense of having a barrel custom turned it's exceedingly foolish to let the length of a FF tube you happen to have on hand dictate what length and gas system you use. Step 1. Define your "mission". Step 2. Decide what barrel length you need to fulfill the aforesaid mission. Step 3. Use the longest gas system reasonable. If it doesn't happen to be a mid length gas system sell the FF tube and buy whatever it is you need. FWIW I would never put a mid length gas system on anything longer than 17". 18" barrels are best with rifle gas, 16" down to 14.5" are beds with mid gas, and shorter than gets car or pistol. Regardless of what the .mil thinks 14.5" barrels run better with mid gas. In your case, if you're completely wedded to the middy idea, I would go with a 16" or 14.5" barrel. "Getting the most" out of your rifle doesn't necessarily mean only the most velocity. Other factors such as maneuverability also come into play. I tend to agree that (in general) a mid gas system is probably a better option for a 14.5" bbl. - vs. a carbine gas system ... That said, however, IMHO neither of those to two is actually the optimal solution for a 14.5" bbl. Not so sure. Dwell length with carbine and 14.5" is about 7" Dwell length with mid and 14.5" is about 5" Dwell length with mid and 16" is about 6.5" Rifle and 20" is about 7" Using gas port hole placement at carbine 7.5" Mid 9.5" Rifle 13" carbine and 14.5" runs good.
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Posted: 1/24/2010 9:47:46 AM EDT
[#32]
When talking about the different gas systems on a 18" barrel, think about the distance from the gas hole to the end of the barrel. The longer that the bullet is in the barrel after the bullet passes the gas hole, the more gas that is getting pushed back through the gas tube. The end result is a sharper recoil impulse.
This is why on a 16" barrel, a mid-length gas system is slightly smoother than a carbine gas system. A mid-length gas system on a 18" barrel is much like a carbine gas system on a 16" barrel, the recoil impulse is going to be sharper due to the longer dwell time. The distance from the gas hole to the end of the A2 flash hider on a 18" barrel with mid-length gas system is approximatley 9.5". The distance from the gas hole to the end of the A2 flash hider on a 16" barrel with carbine-length gas system is approximatley 9.5". A intermediate gas system on a 18" barrel is simular to a mid-length gas system on a 16" barrel. The distance from the gas hole to the end of the A2 flash hider on a 18" barrel with intermediate-length gas system is approximatley 7.5". The distance from the gas hole to the end of the A2 flash hider on a 16" barrel with mid-length gas system is approximatley 7.5" The rifle length gas system on the 18" barrel will be the softest shooting of the 3 gas systems listed. You will find that most run and gun rifle competitors and 3 gunners prefer the rifle length gas system on 18" barrels because of it's softer recoil impulse which aids in faster follow up shots. 
03designgroup | Carbine vs. Mid-Length Gas System on a 16" Barrel 
Keep in mind that the -14.5" with carbine length gas system -16" with mid-length gas system -18" with intermediate gas system -20" with rifle length gas system have approximately the same dwell time (distance from the gas port to the end of the barrel). Click on this link: http://www.bushmaster.com/anatomy_bushmaster.asp - then click on "SPLIT" at the bottom of the screen. Then watch the cycle of operation. Note where the gas port (gas hole) is located. Watch as the bullet travels down the barrel and past the gas port. Think about the information I posted above. If the barrel were longer, it will result in more gas being pumped into the gas tube and into the action of the carbine. More gas equals more force, more force equals a higher recoil impulse. Kind of hard to explain on the internet, much easier to show someone face to face. Hope this makes sense. |
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