A buddy just shortened his 20 inch colt to 16 inches and it is short cycling about 20% of the time..  Any Ideas??  Cycled perfect before the change..  He thinks it may need the gas port enlarged??   Thanks Guys

How did he shorten it - by cutting the barrel?  Why not just buy a new upper or at the very least just a new factory 16" barrel and install that.  

Try to replace the full size recoil spring with a CAR spring, that might help you buddy's problem.  

Quoted: Bushmaster puts the gas port about 4" aft of the muzzle on their Dissipators for good reason.

RAF DOES know the reason and he just said it. The gas port needs to have some barrel out in front of it, in order to build pressure. Imagine if the gas port was 1/4" from the muzzle, It would REALLY work like crap.

I think it "might" help to make the gas hole a bit bigger, but I'm not one to give an exact size. What really needs to be done is move the gas block about 4" back.

The Dissipators' gas block is under the hand guards, about 4-5" back.

If you move the gas block back, you will need a new gas port drilled into the barrel...Be careful that you don't end-up with a burr on the inside. If you put a tight-fitting object (bullet) under the hole you are drilling, it will keep the burr to a minimum. BE SURE TO REMOVE THE BULLET OR OBJECT WHEN YOU ARE DONE. As for the existing gas port, you will need to close it off. Close it off by either tapping it and installing a set screw, or buy a steel ring/clamp type thing that will cover the hole.

Yeah, like Raf said, you just cannot cut 4" off a 20" barrel and expect it to function correctly. There isn't enough barrel past the gas block. That is why Bushmaster uses the 16" barrel for the Dissipator, with the original rearward gas block hidden under the handguard and a forward one for the front sight. I have read about some company (Fulton Armory perhaps?) enlarging the gas port, but feel that it isn't the best way to do it.

Bushmaster Dissipator:

Wow!!  Great input guys..  Thanks..   He saw what we got this far and is impressed..  I think we have a new member coming aboard...   His specialty is handguns..  But he's becoming "hooked" on ARs...

If you shorten a 20" barrel and leave the gas port in the original position, you have to enlarge the gas port to ensure proper function, I believe.  Contact Fulton ARmory.  I think Clint has a FAQ on this.

An other good place to seek the requested information.

www.ar15.com/forums/forum.html?id=44

OSA

No, no, no, no! Don't change the recoil spring nor enlarge the gas port!! The spring will keep the carrier from beating the gun to death, a lighter spring will not keep that from happening.

What he needs is a "pigtail" gas tube. It's a spiral gas tube that holds more gas to allow the carrier to get a better gas charge from a shorter barrel. This may still not cure his ills.

Optimum performance will be gained if you just install a new gas block under the handguards.

AR's require a certain amount of gas pressure at a certain time in  the firing cycle. I do not know the exact numbers, but look at it this way...The gases are at their highest pressure just before the bullet passes the gas port and is about to leave the muzzle. The pressure needed for the rifle to work requires that the bullet be IN the barrel for a little bit after it passes the gas port. The bullet "kinda" keeps the gases in the barrel long enough for the pressure to get to the bolt carrier. As soon as the bullet leaves the muzzle, you have a lot less gas pressure inside the barrel.

Bigyeti,

I think the preferrable way out of this is going to be just buying a new dissapator barrel from Bushmaster and installing it with a new gas tube.  Sure, you could add a new gas port, buy a new gas block, and send it to a gunsmith to do the work and hope for the best, but in the end you still have a pieced together assembly that had to be "fixed" to work right.

There is also the possibility of gas port diameter problems depending upon who you send it to, and the possibility of high or sharp edges around the new port.

Bushmaster lists a new barrel assembly ready for installation at $219.95 which is a reasonable price IMHO.  Sure, it will cost a bit more than "fixing" it, but the savings would not justify the end product IMHO.  I guess your friend can just chalk it up to a lesson learned the hard way.

That's just what I'd do in his situation.  YMMV.


Saleen

IIRC, Kurt at Kurt's Kustom does this sort of thing and uses a modified port...

Send that bad boy to Kurt...

forums.ar15.com/forums/topic.html?id=125505

Thanks a bunch guys...   I'll print this all up for him...  He's a Glock armorer and has some equipment in his small department shop...  This was his first AR15 project.(from parts)  I can't figure why he shortened it without a bit more info..  I'll see him on Tuesday, so if anyone else has any more input, I'm sure he'll appreciate it..   AND I'll probably get some free 9mm ammo out of the deal...Whoopee!!!

I built my girlfriend's birthday gun like that.  I just drilled the piss out of her gasport (teehee) and it function 100%  Email me if you like to talk more.

Quoted: I built my girlfriend's birthday gun like that.  I just drilled the piss out of her gasport (teehee) and it function 100%  Email me if you like to talk more.

 I Emailed you but no answer...   This is what he'd like to try...   I suggested he do it in very small stages and test it after each reaming...

I had a 20" Kurtgewehr'd to a 16" dissipator style so I could handle mount my Elbit Falcon on a 16" carbine.

It has never had a function problem, PERIOD!  On top of that, it was nailing the 200 yard gong at BRC!  Had Kurt attach a fake suppressor for looks only.

I know he modifies the gas port, but I don't know more than that.  I can say that I own one, and it rocks!  It rocks everytime!