How come all 16 inch uppers are not just 20 inch uppers with 4 inches less of barrel?

Yes I saw the Bushmaster Dissipator and that is the right idea. Why was it not done like this the first time?

So for sight radius, it is a no brainer. But that is more reliable gas-wise? A 16 inch full gas system, or a 16 inch carbine gas system?

And is mid-length gas more or less reliable than a full length gas with a 16 inch barrel?

How come the military did not go with a 16 inch barrel instead of 14.5? Since 14.5 is only good to 90-140 yards, I would think 16 would be better for them. I heard  it was because a bayonet would not work for some reason on a 16 inch but I don't get that. They could have done a mid or full-length gas system 16 inch and it would be better.

Gas pressure.  You'll notice there's about 4 inches of barrel after the gas block.  This allows greater pressures to cycle the bolt.

So is that a good rule of thumb? One wants about 4 inches of barrel after the gas block?

Are you saying a Dissipator is less reliable than a regular 16 inch with a short gas system?

If so, I can see that makes sense.

But is there a downside to having the gas block too close to the receiver? Like high cycle rate in full auto?

I am most interested in knowing what works best for the gas system as I will use optics and I can give up sight radius.

Most Dissipators use a carbine gas port under the handgaurds anyway.

Rifle length gas systems can be cut to 17 inches before they start having problems.  Many people has run them at 16 with gas port modifications.

The carbine gas system was developed for the XM-177 with a 10 inch barrel it stuck.  Mid-length and a 16 inch barrel would be an improvement over the M4 in all regards but length and weight.

For function you have to have a certain amount of barrel past the gas port for preasure to remain high enough to cycle. A full length gas system with a 16" barrel would not give enough gas to cycle. The carbine length gas system gives the proper distance to function with a 14.5" barrel. The mid length system places the gas port the same distance from the muzzle as the full length system. Most feel that the mid length system is smoother and more durable than the carbine length with a 16" barrel. The carbine length system with a 16" barrel doesn't work well with a bayonet because the additional barrel length means that less blade extends past the muzzle and doesn't look right. BTW, the Dissipator uses a carbine length gas tube concealed under the handguards, not a full length gas system.

Quoted: How come all 16 inch uppers are not just 20 inch uppers with 4 inches less of barrel? Yes I saw the Bushmaster Dissipator and that is the right idea. Why was it not done like this the first time? So for sight radius, it is a no brainer. But that is more reliable gas-wise? A 16 inch full gas system, or a 16 inch carbine gas system? And is mid-length gas more or less reliable than a full length gas with a 16 inch barrel? How come the military did not go with a 16 inch barrel instead of 14.5? Since 14.5 is only good to 90-140 yards, I would think 16 would be better for them. I heard  it was because a bayonet would not work for some reason on a 16 inch but I don't get that. They could have done a mid or full-length gas system 16 inch and it would be better.

The reason is because there is a history of development behind the 16" barrel.

(note this is a "quick and dirty" overview, someone else can probaly do a much better coverage of this)

Originally some military (Saudi Arabia?  Somehwere in the mideast) contracted out in the early 1970's for Colt to make them some carbines, except that part of the specifications were that the carbines had to be able to mount a bayonet.  Colt already had the carbine length handguards and gas system developed, so basically they extended the barrel from 11.5" until the flash hider was in the right place to mount an M7 bayonet...and that happened to be a 14.5" barrel.

Later on the US military adopted the same length barrel most likley for the same reasons- a more compact barrel that could mount a bayonet.  What you have to realize is that mounting a bayonet is simply a specification requirement of a military rifle.  You can argue "well that doesn't make any sense" but that really doesn't play into the argument- Uncle Sam wants a shorter barrel that mounts a bayonet, and Colt deliverd them an already developed off-the-shelf product.

However, in the United States you cannot have a barrel under 16" without NFA paperwork, so many of the commerical manufacturers just added 1.5" to that length and *poof* a non-nfa carbine barel length was born.  Of course, the bayonet no longer mounted properly, but with the AWB that ended up becoming a moot point.  

The midlength gas system is a very recent development, and wasn't an option back around 1990 because it simply did not exist, not even on paper!  Being what we know now about the carbine length barrels and their ballistics, it would not suprise me if it been available when Uncle Sam was looking for a shorter barrel that it would have been adopted in lieu of the 14.5" barrel.  then again, the military went to the newer 62gr ammunition to be in line with the NATO standard despite the fact its terminal ballistics are inferior compared to M193 55gr ammo.  As you can see, many of the decisions can have less to do with "the best weapon" and more to do with politics and expediency.

As for the Bushmaster Dissipator, it DOES NOT use a full length gas system- it actually has a cut-down front sight block under the handguards, and then a second front sight block that has no purpose in the gas system and is only used to retain the handguards and hold the front sight post.

At this point there is no reason for the military to adopt a midlength barrel from a simple logisitcs standpoint- they woud have to stock THREE lengths of barrel, handguards, and gas tube.  

A more logical route woudl be to shit can the carbine and rifle lengths for 5.56mm

go to 16 midlengths and 12.5" midlength shorties (if needed),  and 7.62mm 20" rifles (Mk11 series guns)

Ok, thanks. I learned a lot.

I have an M4 just cause it is cool but if I were picking an ideal compact  rifle I would probably go mid-length 16 inch barrel.

But since I already have a 14.5 inch M4. I am skipping 16 and picked 18.5 for my current LMT MRP rifle.

I had no idea the Dissipator had two gas blocks.

Interesting.

Ok, how can anyone deny this extra forearm and sight radius is superior?

Quoted: Ok, how can anyone deny this extra forearm and sight radius is superior?

Because it weighs more.   The sight radius only helps when you're using irons and most people are using optics nowadays.

Quoted: Interesting. Ok, how can anyone deny this extra forearm and sight radius is superior?

The Carbean site radius is more than adequate for it's limited effective range.  A decent shooter should easily be able to make consistant 50 yard head shots with the carbean irons.  

I have no idea what value the extra forearm would provide.

Quoted: The midlength gas system is a very recent development, and wasn't an option back around 1990 because it simply did not exist, not even on paper!  Being what we know now about the carbine length barrels and their ballistics, it would not suprise me if it been available when Uncle Sam was looking for a shorter barrel that it would have been adopted in lieu of the 14.5" barrel.  then again, the military went to the newer 62gr ammunition to be in line with the NATO standard despite the fact its terminal ballistics are inferior compared to M193 55gr ammo.  As you can see, many of the decisions can have less to do with "the best weapon" and more to do with politics and expediency.

While the current specification "midlength" may be a relatively recent development, it is NOT a new idea.

There is a Colt prototype 16" mid-length CAR-15 series rifle picture in "The Black Rifle", pg. 170. It has a 3-prong, shortened triangular handguards, and a bayonet lug at the rear of the gas block.

If you use a VFG consistently, the extra tube is less useful.
A longer tube is useful for bracing on barricades, and for grasping its far end when you want the most target to target control.
Unless you're playing games, there is little reason to have a rifle length sight radius vs carbine length sight radius for typical use.   If you need that kind of precision, you should be using an ACOG or MRT instead.

-z

Quoted: Quoted: The midlength gas system is a very recent development, and wasn't an option back around 1990 because it simply did not exist, not even on paper!  Being what we know now about the carbine length barrels and their ballistics, it would not suprise me if it been available when Uncle Sam was looking for a shorter barrel that it would have been adopted in lieu of the 14.5" barrel.  then again, the military went to the newer 62gr ammunition to be in line with the NATO standard despite the fact its terminal ballistics are inferior compared to M193 55gr ammo.  As you can see, many of the decisions can have less to do with "the best weapon" and more to do with politics and expediency. While the current specification "midlength" may be a relatively recent development, it is NOT a new idea. There is a Colt prototype 16" mid-length CAR-15 series rifle picture in "The Black Rifle", pg. 170. It has a 3-prong, shortened triangular handguards, and a bayonet lug at the rear of the gas block.

Well damn, just give me another reason to go buy a copy of "The Black Rifle."

Maybe I'll just have to cut down some triangular handguards and make my new middlength a clone of that!

Quoted: If you use a VFG consistently, the extra tube is less useful. A longer tube is useful for bracing on barricades, and for grasping its far end when you want the most target to target control. Unless you're playing games, there is little reason to have a rifle length sight radius vs carbine length sight radius for typical use.   If you need that kind of precision, you should be using an ACOG or MRT instead. -z

Also less exposed barrel to burn you if/when you transition to your sidearm and let the rifle drop against you.  Good for some, irrelevant for most.

Quoted: go to 16 midlengths and 12.5" midlength shorties (if needed),  and 7.62mm 20" rifles (Mk11 series guns)

Do you mean a 12.5" with a midlength gas system?  What sort of gas port work (size) would be needed for that?  I think you mentioned a 12" or 12.5" 6.8 upper before, but I hadn't thought about a shorty midlength 5.56.  [:\]

Quoted: A more logical route woudl be to shit can the carbine and rifle lengths for 5.56mm go to 16 midlengths and 12.5" midlength shorties (if needed),  and 7.62mm 20" rifles (Mk11 series guns)

HOT DAMN!!!  That is a good idea if I ever saw one.  Long range hits, short range kicks, and 16 inch general purpose that does a little bit of both and does them damn well.

Quoted: Quoted: The midlength gas system is a very recent development, and wasn't an option back around 1990 because it simply did not exist, not even on paper!  Being what we know now about the carbine length barrels and their ballistics, it would not suprise me if it been available when Uncle Sam was looking for a shorter barrel that it would have been adopted in lieu of the 14.5" barrel.  then again, the military went to the newer 62gr ammunition to be in line with the NATO standard despite the fact its terminal ballistics are inferior compared to M193 55gr ammo.  As you can see, many of the decisions can have less to do with "the best weapon" and more to do with politics and expediency. While the current specification "midlength" may be a relatively recent development, it is NOT a new idea. There is a Colt prototype 16" mid-length CAR-15 series rifle picture in "The Black Rifle", pg. 170. It has a 3-prong, shortened triangular handguards, and a bayonet lug at the rear of the gas block.

I saw this and thought that it looked like a middy, but no one else had ever said anything about it.

I am unsure of the gasport size for the 12.5's

Steve Troy made a few - and so have others.

Perhaps Wes Grant or someone could chime in.

....... A full length gas system with a 16" barrel would not give enough gas to cycle.

I took my Colt HBAR way back in the late 80's and cut it to 16.1", turned down the barrel profile and fluted the barrel. Way before the any Dissipators were release by the real builders. I just wanted a shorter / lighter barrel using what I already had. The Gas port was left stock, and it has worked fine with pretty much any ammo for the last 15+ years. I guess dumb luck or good karma on my part.
But I will say, the 'new' mid-length set-ups are now by far my favorite for use, looks, handling, etc.

We used to have 20" barrel rifles chopped back to 16"--they worked perfectly, as long as the gas port was opened to around 110 or so.

Quoted: We used to have 20" barrel rifles chopped back to 16"--they worked perfectly, as long as the gas port was opened to around 110 or so.

Thank you Doc.  I appreciate you taking the time to add to this discussion.

Dawg180 is on the right track.  The 16" barrel with carbine length gas system is a product of a combination of the gun laws and the developmental history of the AR15.  Dawg180 didn't go back far enough though in his history lesson.  The first carbine design was the XM177 with 10" barrel.  The handguards and gas system was designed for this length of barrel.  However, this barrel length proved problematic and Colt later lengthened the barrel to 11.5".  

Due to the NFA, civilians could not easily own a XM177, so Colt lengthened the barrel to 16" for civilian sales.  It was after all this that the M4 was developed which took the XM177 design and lengthened the barrel so that a bayonet could be mounted.