I appreciate the many concerns that could (and probably will) be expressed about the undertaking of such a project.
This is not intended to encourage the average hick with a hacksaw to ruin his rifle.

If you don't have experience and access to machine equipment, or have to do a web search to find out what "port velocity" or "column inertia" is you probably shouldn't try this yourself. There are many reliable gunsmiths out there that work for reasonable rates (and will probably save you money in the long run if you see one first)  

That having been said.

I can acquire a couple of .625" (5/8) diameter  20" barrels for pretty short money.
I would like to cut one down to 16" and build an inexpensive dissipator.

My biggest concern deals with the rifle length gas system and making it function properly.

I have had a couple ideas of improvising an adjustable gas system but even in theory they seem likely to become more troublesome than they are worth.

I have no usable data, reference, or starting point for tuning the gas system outside of the current configuration.
Opening up the gas port a couple thousandths at a time using trial and error as a guide could become very time consuming.

What I really need is someone who has practical experience/knowledge about making this modification work.
If you have had success please leave detailed specifications along with any insight barring the before mentioned.

My suggestion would be to get a low profile gas block that can be fitted underneath the HG's using either a Mid length or CAR length gas tube and seal the gas port in the original FSB.

No real experience here but I'm reasonably sure that with the Gas port  so close to the muzzle that the pressure spike required to operate the BCH relaibly will be difficult to achieve.

Of course I could be full of sh**

Jason

you're full of shit.  Why take the cheap route like Bushmaster and add weight to the rifle?

Fulton Armory does it correctly.  The gas port must be enlarged.  Somebody will be along shortly to give the specs.

Jason :

Don't be hard on yourself.

I want to thank you for your suggestion,
(it is certainly better than the negative responses I half expected to see posted)

it may be a feasible option for some readers of this thread who are looking into starting a similar project.

It is one of the earlier avenues I had considered, at first glance it does seem quite simple and nearly foolproof however after further consideration I decided not to go that route, at least not at first (it is a good contingency plan though) My reasoning is as fallows:

1, You haven't really gained much ground, you still have to tune this new gas system because the new gas block isn't going to be pre-tuned "bolt on" for this exact application. There are simply too many variables to consider, most importantly the new port you have to drill in the barrel to accommodate it but also the different barrel length, and where on the barrel it is mounted.The manufacturer can't account for all of these.

2, Sealing the gas port in the original front sight base still leaves a void in the barrel itself, lacking any dynamic flow through it, it will inherently collect all manner of nastiness including moisture when the rifle is not in use.

If all else fails I may still go that route but I will personally like the rifle less for it.

m193:

I can see you know what kind of information I am looking for,I want to thank you and I'm going to give Fulton Armory's website a good look over maybe even call them to see if I can get someone knowledgeable on the phone (as unlikely as that is)

I do hope this thread will help others that are looking into similar projects so I don't want to deter anyone from making contributions that somebody else might find useful.

Don't get me wrong I personally feel that nobody is above constructive criticism, so by all means if you see someone is taking the wrong route help redirect them, don't just watch and laugh as they get lost.

send it to adcofirearms and tell him to cut it down and open the gas port so it would function properly.

Quoted: send it to adcofirearms and tell him to cut it down and open the gas port so it would function properly.

+1

Adco really knows their stuff.

I thank you all for the gunsmith recommendations.
I am sure they will be excellent choices for those who can not, should not, or have no desire to perform the work necessary to complete a similar  project themselves.

I would still like to hear from anyone who has specific knowledge of the modifications performed.

I hope I have not placed anyone under the impression that I want them to give me a definitive "this will work on your rifle". I am interested in what has worked on another similar rifle. The intension is to gain a reference point, like when you work up a hand load (you don't start at someone else's maximum pressure load, you start below it and work up to where they are)

Increasing the port diameter a couple thousandths at a time starting at the factory dimensions  could become very time consuming, however starting a couple thousandths below a "known good configuration" would be very helpful in saving time.

no harm no Foul as far as I'm concerned. Like I said in the original post "I could be full of Sh**"

Anywho, thanks for your kind words and you've obviously put more thought into it than I have but then again I've never been a fan of the Dissy front ends.

Jason

I think I have figured out a good way to find a reference point biased the barrels original configuration.
If you see a problem with the theory, my equations, or estimates please let me know

Though this looks complicated I assure you it really is not, its just long-winded...

We know that the barrel (which includes the gas port) has been engineered to function properly at the standard 20" length.
The function of a properly engineered gas system is to apply the appropriate "power" to cycle the bolt/carrier assembly.

So here is the math:
The rate at which energy is added to a system (energy divided by time) is called power. (Mass is represented by "m" Velocity as "v", Time as "t", and Force "F"). With mv divided by t, power becomes force only (mv/t = F). With ½mv² divided by time, power becomes force times velocity (½mv²/t = Fv).

Now we can call most of the properties we are working with constants :
Which is to say that they are unchanged by the decrease in barrel length past the gas port.
The bolt assembly still requires the same force (F) to cycle it, the pressure in the barrel at the gas port is unchanged because the volume of available gas exceed that required to supply the appropriate mass (m) , the velocity (v) at, and acceleration of the bullet and gasses past the gas port for the duration of time the bullet remains within the length of barrel to the point where the shorter barrel terminates.

The only part of the equation we need to solve for is time (t):
To figure out the difference in time that force is being applied to the bolt assembly between the two configurations is a mater of determining the velocity and rate of acceleration of the bullet starting at the point where the shortened barrel terminates and ending at the point that standard length barrel terminates this product is the amount of time it takes the bullet to travel the difference in barrel length. This could potentially lead to a bit of math so lets take a shortcut, I have looked at a few test charts and it appears that a 4" difference in the barrel at these lengths only amounts to a about a 5% difference in terminal velocity. Where terminal velocity is only reached literally at the point that the barrel terminates and takes the entire 4" to accelerate to that point (this amounts to a negligible difference as it is within "commercial accuracy" and there is greater variation in the different ammunition the rifle is designed to fire)

What this means to us is that you can simply divide the length of the barrel that extends beyond the gas port on the shorter of the two, by the same measurement on the standard barrel (which is 4" longer) the product of this equation is the difference in the value of (t)

By decreasing the value of (t) as it applies to the difference between the shorter and longer barrel configurations we can solve the new value of (m) while retaining the constant value for (f).

Now we know how much we need to increase the value of (m) to apply the proper (f) to cycle the bolt assembly in the (t) we have available.

We could discuss port dynamics at this point, however it once again could lead to a bit of math and unless I am mistaken the variation we would find would also fall within "commercial accuracy" best of all this variation acts opposite the the one we allowed in figuring (t) (meaning they actually work together to help cancel each other out, making the end product of the two estimates more accurate not less)

Because the pressure inside the barrel at the gas port is unchanged we simply increase the aria of the port opening relative to the difference in the value of (m) between to two configurations (which is directly proportionate to the change in the value of (t))

Important note:
We are changing the value in the aria of the opening. (though this will obviously reflect as a change in diameter "aria" and "diameter" are not interchangeable in this equation) so we will have to do some math: πR² to find the aria of the opening biased on its diameter.

I ordered the barrel today so I don't currently have the diameter of the gas port handy, but once I do I will use it to illustrate the equation including the aria translation of the factory port diameter, the total aria after the required increase, and its conversion back into the new diameter.

I am going to conclude with the statement that this (in theory) should be fairly accurate however I still plan to start with a port diameter smaller than that provided by this equation, for the simple reason that if the theory or equation are flawed and you end up with too large a port you can't just make it smaller...

Quoted: Though this looks complicated I assure you it really is not, its just long-winded... ..... I still plan to start with a port diameter smaller than that provided by this equation, for the simple reason that if the theory or equation are flawed and you end up with too large a port you can't just make it smaller...

All that and your still defaulting to trial and error?  

Forget your math.  20" barrels chopped to 16.25" need a gas port of 0.105-0.110", in my experience.  I get success with all ammos except Wolf at 0.105".  I've done this four times with Colt 1/7 gov't profile barrels, which have a gas block area profile of 0.750".  I'm not sure if the old A1 profile barrels with the 0.625" gas block area profile area would be much different, but I'd guess it wouldn't much.

UVvis:

Not everything that looks good on paper comes out exactly as planned, and just because I'm the one who wrote it down, doesn't make it the exception to the rule in my eyes.

Besides I have only been looking for some good reference figures like those supplied by mongo001, which by the way I would like to thank him for.

The larger the data pool I can draw from the more likely it is that my starting point will be close to the finishing point. That is all I have really been looking for, to save some time.

If there are any more people out there that have usable figures from first hand experience like mongo001 please feel free to post them regardless if they are identical or not (either way they will be useful).

Anyone who has found a flaw in my theory or has another feasible equation I would be glad to hear from you as well.

my dissipator is a 20" hbar cutdown to 16. A gunsmith did it, not me so thats all i can tell ya

btw, it is reliable with all ammo except for WOLF

christ0ph:

If it wouldn't be too much an imposition, could you ask your gunsmith the next time you have the chance, and post the the figure you are supplied?

Even if you don't get around to it before I finish my project it may be useful to others who are looking for this sort of information for their projects.

Mongo:

I have clipped several off @ 16.5 inches, and have always had good results doing the following:

1) Drill to .1110 (#34) diameter

2) Finish ream to .1142 (2.90 mm) diameter.

They will run anything from M855 to Federal 40 grain stuff on a registered lower.  Never had a complaint, and the rate looks good too.


Lem

Lem:

If you don't mind me asking, where all the barrels the same diameter at the gas port, if so what diameter where they?

By the way I like your idea of using a reamer to finish, it would make a much cleaner port and likely eliminate the chance of leaving a bur inside the barrel.

Trust Mongo! Mongo Knows!   (I'm not being facetious - some of the regulars on this forum love nothing better than to flame and pontificate. After a while you get to recognize the gurus from the posers). Mongo's no poser.

I had a 20" gov't profile cut to 17" by Wakal.  I took it out for testing and it would not lock the bolt back on standard pressure loads, but it would with NATO stuff.  I pulled the FSB , opened to 0.106", and it functioned 100% with anything from Wolf to XM193.  

I didn't ream the hole.  After reading Derrick Martin 's(Accuracy Speaks) book, I wasn't too worried about the burr.  I cleaned it and shot off the burr.  I figured if it worked for his competition guns, it would work for the beater truck gun.

So far it has been a great setup.

Quoted: Trust Mongo! Mongo Knows!   (I'm not being facetious - some of the regulars on this forum love nothing better than to flame and pontificate. After a while you get to recognize the gurus from the posers). Mongo's no poser.

+1

I learned a lot from him before I was even a member here. When it comes to not so ordinary mods., Mongo is one of the "go to guy's"!! Case in point, he just taught me something else right here I did not know and now I am going to use it to my advantage and hack up a perfectly good 20" bbl.!

Thanks Mongo!