OK,
So I have my M4 clone all ready to go for Sept 13th.  Now comes the question of what to do with the other Ameetec lower I have on the way.  I really think a "Dissy" would be cool.  I was thinking something along the lines of a A3 Dissy with LMT BUIS, Cavarms non-tapered HG (or maybe a rail), and Vortex or Phantom.  Lately, I've seen Mid-length systems talked about.  What are the advantages of the mid-length systems as opposed to the "Dissipator" design?

Or should I just build a "traditional" A2 with it?


Rich

The Dissipators have the advantage of longer sight radius.

The practical strength of the mid-length is the gas system.  The longer tube allows a softer pulse into the system.  They take a bayonet and have a more conventional appearance.

As far as I know no one makes a Dissipator with a mid-length gas tube.

I chose a mid-length, but it really comes down to personal preference.

Trumpet, just buy both!  Just kidding.  I'm also trying to decide between the two.

i'm doing a dissy setup very similar to that with a collapsable stock. i like the fact that it's short with a long sight radius.

I have a Dissy upper, and when I mate it to a tele lower, it seems to be front heavy. Sooner or later, I will send it off and have it fluted to balance it out. YMMV JMHO-AMOS  

Quoted: I have a Dissy upper, and when I mate it to a tele lower, it seems to be front heavy. Sooner or later, I will send it off and have it fluted to balance it out. YMMV JMHO-AMOS

Yep, I tried a couple telestocks, ended up with this....

I like the dissipator with the CavArms non-taperd hand guard.

dot-bob,
I really like the looks of the non-tapered hg's.

Underdog,
Please give me all the particulars about your rifle.  Nevermind, I just saw your reply on the other "Dissy" thread.  It's awesome!

Rich

Quoted: As far as I know no one makes a Dissipator with a mid-length gas tube.

Made up a few batches of them ealier in the year... this is my personal favorite to tell the truth, mid-length system dissy.

Gunzilla,
Are you a gunsmith or do you work for a manufacturer or do you just "tinker"?  Why does know one make a mid length gas system Dissy?  Sounds like it would be a gem!

Rich

I was a manufacturer, until the whole DHS got all spun up and I was forced to put all that on hold until peace breaks out again I know several other manufacturers that I would be happy to fill this gap... if there was a good market?

The ones I offered were on a 16" SS barrel, 1:8, and had a choice of standard FSB (sort of standard, it can be slipped off over the flash hider... so you can get the handguards off) or a railed block or no front sight.

We also were going to run a batch of chromo steel barrels with NiB plated bores... if the interest is there, who knows?

Quoted: Quoted: As far as I know no one makes a Dissipator with a mid-length gas tube. Made up a few batches of them ealier in the year... this is my personal favorite to tell the truth, mid-length system dissy.

Neat!  I always felt that this made more sense than the 'normal' Dissipator system.

'Zilla,
You know of anyone that makes 'em still?  Howzabout in a chrome lined bore?  I wonder why Bushy didn't go with a mid length gas system in their Dissy?


Rich

Midlength is great but if I were to do it agian, Id go with a cut down 20" govt profile instead, KKF does it Im sure,.. FWIW...........Udog

I am calling a friend today on these... as for me, I would prefer NiB to chrome plating, but... I guess what we were offering earlier this year would be the most popular? 16" 1:8  threaded barrel, Wylde Chamber, .750 profile, mid-length gas system and rifle length sight placement.

Quoted: Midlength is great but if I were to do it agian, Id go with a cut down 20" govt profile instead, KKF does it Im sure,.. FWIW...........Udog

IMO... a cut down 20" barrel requires that the gas port be opened too far, we played around with cutting A1 barrels back to 16" back in the early 80s and gave up on it then. The dwell time of a mid-length system is about perfect, based on the port pressure and oal -- even in extremely cold operating environments, the mid-length runs like a top.

Quoted: Quoted: Midlength is great but if I were to do it agian, Id go with a cut down 20" govt profile instead, KKF does it Im sure,.. FWIW...........Udog IMO... a cut down 20" barrel requires that the gas port be opened too far, we played around with cutting A1 barrels back to 16" back in the early 80s and gave up on it then. The dwell time of a mid-length system is about perfect, based on the port pressure and oal -- even in extremely cold operating environments, the mid-length runs like a top.

There's a couple guys running cut down 20's (Troy come sto mind) But I have no personal experence with them and I was still pooping green in the early 80's so I'll bow out on this one,  I do have middy experence and will agree that its a smooth system without a doubt.  Udog

Quoted: Quoted: Quoted: Midlength is great but if I were to do it agian, Id go with a cut down 20" govt profile instead, KKF does it Im sure,.. FWIW...........Udog IMO... a cut down 20" barrel requires that the gas port be opened too far, we played around with cutting A1 barrels back to 16" back in the early 80s and gave up on it then. The dwell time of a mid-length system is about perfect, based on the port pressure and oal -- even in extremely cold operating environments, the mid-length runs like a top. There's a couple guys running cut down 20's (Troy come sto mind) But I have no personal experence with them and I was still pooping green in the early 80's so I'll bow out on this one,  I do have middy experence and will agree that its a smooth system without a doubt.  Udog

No big deal, I know a few guys that do the 20" cut down jobs... in talking to them I just got a few things that I did not dig; "we open the port to .110" -- That is half the diameter of the pill and nearly twice what the rifle *can* run at, "they shoot fine... most of the time"-- all righty, I just tend to live in the *rest of the time*, "works good with full power ammo" -- these guns, to me, are tools and I prefer the reduce the limitations of my tools working to zero.

I have owned and shot a lot of cut back 20s and they work out okay to tell the truth, but I see it as not as efficient as the mid-length system... I don't mind taking the time and effort to make things as close as they can be to right on, I have 5 different gas port locations to use for different lengths of barrel, only three of them are common enough to get barrels "off the shelf" but thankfully the mid-length is one of them, it has been accepted for good reason.

I have A cut down '20 Dissy.
It runs fine...Tho I have never really had the opertunity to try it out in really cold weather.

There is always the 18" barrel with a full length gas system (SPR) option.

I have these two. They both have "H" buffers with M16 carriers. The 16" lightweight midlength barrel does have a different recoil impulse than the 16" M4.

Gunzilla,
What exactly is NiB plating?  Not to sound too ignorant, but what is the difference in "pulse" you all are referring to?  Does the longer gas tube make the weapon more reliable/more comfortable to shoot? What would something like you're talking about cost (roughly)?

As to Dissipator barrels and mid-length gas systems.  Why doesn't anyone make a Dissy type barrel (long sight radius) with a full length gas system?  I would think that would be the best of both worlds.  I mean if they make 16" barrels with mid-length gas systems, why not just make it full size?

Thanks,
Rich

Quoted: Gunzilla, What exactly is NiB plating?  Not to sound too ignorant, but what is the difference in "pulse" you all are referring to?  Does the longer gas tube make the weapon more reliable/more comfortable to shoot? What would something like you're talking about cost (roughly)? As to Dissipator barrels and mid-length gas systems.  Why doesn't anyone make a Dissy type barrel (long sight radius) with a full length gas system?  I would think that would be the best of both worlds.  I mean if they make 16" barrels with mid-length gas systems, why not just make it full size? Thanks, Rich

For the technical reasons why the mid-length tube is better than the carbine length, better see this article:
Armalite:  Effects of Barrel Design on Reliability

In summary, Armalite found that the mid-length system is just right for a 16" barrel.

The idea of a full length gas system on a 16" barrel has been tried before.  Tests found that this modification compromises reliability because the port is so close to the muzzle.  This makes the gas pulse too short.  Opening up the port size was not sufficient to solve this issue, hence the use of the shorter gas tube.

Ahhhhh.
Thanks.  If I get a mid length upper, is it possible to have the FSH moved forward so the upper can accomodate full length HG's and rails and also get the longer sight radius?

Thanks
Rich

Trumpet,

A rifle length gas system wouldn't work with a 16" barrel because there wouldn't be enough barrel in front of the gas port to allow for needed dwell time. That's the reason the dissy has another gas block under the handguards.

Innocent Bystander,.
I'm aware of that...now.  I was wondering though if a FSH could be moved forward on a midlength, not for the longer gas system (after all it's already mid-length) but to take advantage of the longer sight radius.

Thanks again,
Rich

okay... NiB is Nickle Boron, a few people think it is superior to chrome as a bore plating.

People have been moving the gas port around on the barrel for as long as the rifle has been around, but here is a quick run down -- some of this is my opinion and I know there are people in the industry that do not agree with all of it, but have not gotten a good argument against it yet.

The original rifle uses a 20" barrel with the gas port at 13" (these figures are *close enough*), during the development of the XM177/Commando/Carbine the barrel was cut to lengths from a little more than 10" to 11.5" for these rifles, the gas port was moved to 7.5" -- as you can see from the chart on the Armamlite page, the port pressure at 13" is around 13,000 psi and nearly double that at the 7.5" position. What is a more important measurement really, is the peak pressure inside of the bolt carrier, which is again nearly doubled for the short (carbine) gas system v. the long (rifle) system.

The pressure at the port is not the only thing in play here, the amount of time (dwell) that pressure exists is a major factor... the original rifle design has 7" of barrel past the gas port to determine dwell -- along comes the M4... in a quest for a shorter barrel (the XM177 project was determined unreliable), here is were that 7" of barrel comes in to play again, but this time for a different reason entirely.

That 7" is also what is needed to mount a bayonette to the standard FSB lug and flash suppressor, Colt already had the tooling and production for a rifle with a gas port at 7.5" (from the carbine)... so, if you add 7" to that (for the bayonette), well guess what the barrel length would be?

That part is my mind at work, I am told that the rifle was specced with a 14.5" barrel and it all happened to just work out really well...

Now, here is one of the first problems, with the increased gas pressure at the carbine length gas port, the dwell time becomes long and the rifle is trying to unlock early... bolt velocities are also way up there (pressure inside the bolt carrier are now very high). I prototyped a rifel at the SHOT Show last year that some may have seen, that uses a mid-length gas port on a standard M4 barrel (14.5), mounts a bayonette and GL and still uses carbine handguards and all the doo-dads.

The mid-length system moves the port to 9.5" with 6.5" of barrel (dwell) past the port... this system works so well with a 16" barrel it is not funny.

We made some dissy stlye uppers using 16" barrel and mid-length gas system, they were very nice little field carbines, the only downside was that you can not mount a bayo or telescoping suppressor to the gun... the alternitive to moving the gas port position is to make it smaller or larger so the gun fucntions -- Very few shops will bother to take pressure readings (or even do computer genterated models) of what the operating pressure of the rifle is in a particular setup. The internal bolt pressure should run 1 to 1.5 Kpsi -- For you gear heads think of it like this, GM builds a motor that is designed to run with a 8:1 compression ratio, then someone comes along and stuffs a supercharger on it without making any other mods, sure it will still run, just not quite right (and probably not near as long). Moving the gas port around is a bit of work granted, but i maintain that it is the best way to get things running right.

So, you got the 3 common lengths, carbine - mid - rifle, that have been accepted in the industry. We deal with 2 other locations for specialized rifles as well... anyway, hope it helped someone?

The rifle in my pic is exactly that, Mid length gas system with the FSB moved forward.
I used a clamp on PRI FSB as the profile at the end is .740, ...Fulton Armory uses clamp on standard FSB's I think, Maybe you can use one of those FWIW....Udog

Quoted: Ahhhhh. Thanks.  If I get a mid length upper, is it possible to have the FSH moved forward so the upper can accomodate full length HG's and rails and also get the longer sight radius? Thanks Rich

sure, looks just like a regular dissy

Thread 1
Thread 2
Thread 3
Here's 3 threads that I ran on my rifle maybe some of the info helps you, Alot of the pics are gone but if theres any thing I can show you just ask...........UDOG

Thanks Gunzilla and Underdog_75,
A "Dissy" with a mid-length gas system sounds pretty much exactly like what I want.   Sorry for all the cross-posting (I posted very similar questions in several forums in search for the answers).  Is moving the FSB forward a relatively simple thing pretty much any smith familiar with AR's can do?  Also, would this be possible with the group buy barrels that they're trying to get going (the 1:7 chrome mid length)?

Thanks so much for all the help!

Rich

Quoted: Quoted: As far as I know no one makes a Dissipator with a mid-length gas tube. Made up a few batches of them ealier in the year... this is my personal favorite to tell the truth, mid-length system dissy.

A mid-length Dissy seems to be a great combo!  I wonder why the big gys don't offer one?

Quoted: There is always the 18" barrel with a full length gas system (SPR) option. I have these two. They both have "H" buffers with M16 carriers. The 16" lightweight midlength barrel does have a different recoil impulse than the 16" M4. im1.shutterfly.com/procserv/47b4d611b3127cceb52bec6afe4d0000001610

Who makes/where did you get the barrels?

Quoted: There is always the 18" barrel with a full length gas system (SPR) option. im1.shutterfly.com/procserv/47b4d611b3127cceb52bec6afe4d0000001610

I am  very interested in the way the SPR system works . Anyone care to educate me? TIA

Hey guys,
I've been talking to Steve at Adco to see what can be done here.  Do any of you know of the quality of Fulton's clamp on FS?  

Underdog,
How is the quality of the PRI FS?  Is it meant to clamp on to the .740" instead of .750"?  Or does it sorta happen to work out?  

Thanks
Rich

Quoted: Hey guys, I've been talking to Steve at Adco to see what can be done here.  Do any of you know of the quality of Fulton's clamp on FS?   Underdog, How is the quality of the PRI FS?  Is it meant to clamp on to the .740" instead of .750"?  Or does it sorta happen to work out?   Thanks Rich

Quality is outstanding,.... A tad bit of horizontal movement when deployed, But not much.....Its meant to go on the standard .750 but theres enough clamping to fit the .740 turndown area like it was meant to be,.... Dont hold out on us what did Steve say?

Underdog,
Check your IM.

Rich

Here's a thought.  
Would a "standard" A1 FSB (.625", I believe) work if I just had it "Opened up" to .740"?  Hmmmmmm


Rich

Gunzilla,
Check your IM

Thanks
Rich

I was planning on having a rifle cut down to a dissy. I think I need to reread this post a couple more times to fully grasp what has been discussed.

Quoted: Here's a thought.   Would a "standard" A1 FSB (.625", I believe) work if I just had it "Opened up" to .740"?  Hmmmmmm Rich

Only if theres enough "meat" there to ream............Udog

I was thinking the same thing.  Steve's checking on it and will let me know.  My fingers are crossed.

Rich

Bigbore,
'Would that be less expensive?  That would be great if that could be done to one of the RRA Midlength chrome lined barrels!

Rich

God damn,
The answer's been right there the whole stinkin' time, right in front of me.  Duh.

WooohOooooooo! When I'm all set (have the funds together) I'll be givin' youse guys a call


OMG!  Even Bigger DUH!  I just realized that I don't need the barrel "turned down", I need it turned "up", which I really don't see as being too effective.  Walt Krulek recommended using one of the clamp on FA ones and "shimming" it if needed.  Good idea or bad juju?


rich

(edited for realizing what a dumbass I was when I posted earlier )

You meant turning down the .740" barrel enough to accept a .625" FSB right?  Would a Phantom still be able to be fitted to the muzzle then?

Thanks.....again

Rich

I'm still trying to decide whether or not to have ADCO build me a dissipator look using a mid-length gas system and SS 1:8 barrel...could be fun.

What is the barrel life of the SS 1:8 like compared to a chrome lined 1:9?  The SS may be the default route to go if I can't figger something out.  So far the chrome lined 1:7 with the front of the barrel contoured down to .625"/A1 FSB/ KAS FF RAS (or CavArms non-tapered HG's)/Phantom looks like it may be the way to go.

Rich

Also,
What's the accuracy potential of a non-FF Dissy barrel?  I was also toying with the possiblity of a KISS Dissy with the CavArms non-tapered HG's.  However I ass-u-me that even a stock barrel will be more accurate if it's FF than not?

Rich

Quoted: ...here is a quick run down --

Excellent and informative post.

Thank you Gunzilla.