I just got done with a deal that I thought was for a 16 preban dissipator complete upper.I was surprised to find upon removing the handguards that it was actually a 20"USGI profile barrel that had been cutr down to 16".I would be mad but I don't have that much in it and it looks absolutely flawless and the chrome at the crown is not chipping so it looks like it might work anyway.It also has the added advantage of the Gov profile under the handguars to mount my M-203.Has anyone heard of severe reliability problems with this configuration?I read an article years ago about MACVSOG cutting down some std length M-16s to make a campact greandiers weapon but I don't know how long they would function or if they needed gas port modifications to be reliable.If any of you guyd know the nuts and bolts of these gut down std barrel please fill me in.I really like the idea of being able to QD my M-203 but have a full length sight radius and full size M-203 handguards.



Thanks


SOPMODM4A3


BTW:would an adjustabel gas tube help to compensate for gasport erosion down the road?

The only problem is in FA fire. The diameter of the gas port would need to be .062 instead of the .092-.001 in the M16A2 for reliable functioning.
BTW-I am going to make up a LW carbine the same way. The dissipator style, with a cut down barrel and an A1 length stock is a true carbine in the traditional meaning of the word. Unlike the Car15/M4.

Thanks Stg44,I feel a little bit like I got "hosed" on this deal but since the gas port would need to be smaller to assure reliability I guess I could get one of those adjustable gas tubes to compensate(cut the flow a bit).Do you think that would work?



Thanks


SOPMODM4A3

First and foremost, I have to ask the question: "How does it shoot and function?"

If it's fine, then don't mess with it.  

If it's not, then I'd suggest you see Tweak in the Troubleshooting section.  

If you're not sure, then grab a case of quality ammo and find out.

Let us know how you do.

Pardon my ig'nance, but wouldn't the port have to be larger in a shorter barreled rifle...

I would think that the port would have to be larger in a shorter barrel as the duration of the pressure would be smaller, hence the need to allow more gas through???

I took my 20" Colt barrel and had it cut down to 16" and re-threaded.  Yes, you have to pay attention to the chrome plating when doing this.  I had it cut on a lathe, crowned and then stoned.

I had to cut about 1 1/2 coils off the buffer spring to get it to function reliably at the start, but now it's flawless (semi auto only, no class III [:(] ).  It might work without removing any coils now as it was also a new bolt/bolt carrier that went in at the same time and I used the McFarlen ring.  I remember it being a little stiff going in the first time.  it might be a little more broke in now.  Seems to work with a multitude of different loads as well.  I save all my reloading experiments and left overs from matches and load up a Beta and they all just rip through it.

The smaller gas port can increase the speed of the flowing gas like a venturi.It's a balancing act between gas volume and gas speed to assure flawless functioning(espacially over the long haul).I just shot it this morning and it functioned OK for a full 30rd mag(semi only)but my concern is for how long.I know that gas port errosion would kill the old shorty CAR-15s because there was so little barrel forward of the gas port to help it generate the gas volume required to operate the weapon.I need my stuff to last for 10,000 rds because of the amount of live fire training that I do.I guess I could just buy another barrel later when I have some more money and just use this for training until I trust it.



SOPMODM4A3

The gas port diameters are directly from the Colt Armorers manual. And they are up to date as of June 2000 when I attended the course.
BTW, we (the department) have two BM M16A2's. They came with a specified 16" barrel. They would not function on F/A for shit. I kept telling the SWAT folks it was the gas port diameter. They changed bolts, buffers, and on and on..finally I put my M4 upper and A2 upper on it. They ran like champs. Semi auto is not such a problem.

Kurt of KKF and Fulton Armory both produce or convert uppers to this configuration.  Both say they work fine if you enlarge the gas port.  Fulton Armory goes on to say that this configuration with the enlarged gas port will erode more slowly than the standard gas port placement on both 20" and 16" barrels due to slower gas speed entering the port.  I have thought about this configuration for a while and am still considering it depending on the availability of funds.

sopmodm4, let me know if you decide you don't want to keep it.  Depending on what the twist rate and the price I might be relieve you of your worries.