I was looking into making a 16in barrel with a rifle length gas tube and was just wondering how big the gas port ID would have to be to get it to cycle 100%?

I know there are many people here using that setup and now KKF builds these too.

What are the advantages of this over standard carbine tubes?

I doubt anyone gets these working 100%.  A gas operated rifle like the M16/AR15 needs a certain volume of gas at a certain pressure to operate as designed.  With a short barrel length after the gas port you can probably get the volume, but it'll have to be at a higher pressure because the time duration of the pressure pulse is less.  

This is why the reliable Dissipator uses the same gas port and tube as the 16" carbine.  You trade a one or two once gas block for reliability.

-- Chuck

The nice thing about these is that they aren't burying a short gas tube under a FF RAS.  I could change a rifle length tube easilly without removing the front sight post/gas block.

Also getting less debris (as more powder is burned at 14inches than 8.5)  into the B/BC.

I just put together a really nice upper with BM Dissy fluted HBAR and KAC FF RAS, I used a KAC low profile gas block and PRI front sight (then when I got it all together i realized how much time it would take me to change out the tube if the gun didn't take well to the transplanted PRI Fatboy gas tube.)

I think accessing the rollpin is no problem through the FF RAS, but the Fatboy tube can't be removed without moving the gas block forward. (which requires access to the two allen screws on the bottom of the block.) Basically making tube replacement require, an allen wrench, vise, vise blocks, hammer, punch strap wrench and about 1.5hours. (in the Army that would be tier II or III maintenance and probably more importantly in my house it is a big PAIN IN THE ASS).


If it functions 100% I'll probably never have to worry (I doubt a Fatboy will easilly burst), but the same tube jinxed my last settup (which was why I just had to see if it would work with this rifle.)

[b]Make a long story short- the setup complicates this whole maintenance operation, I just think a rifle length tube is a better idea for field use (provided it can be made to function properly.) [/b]

Green0,

Each one is a little different. You'll have trial and error it. You don't want to go over 0.125" as that's the ID of the port in the FSB. It'll most likely be larger than .104 if you're using a .75" diameter FSB seat.

From experimenting in the shop we never had good luck with 16" .75 OD barrels. Shortest we could get, and hold reliability, was 17". With the older .625" OD barrels we could go down to 16".

Cut the barrel to length, then load single rounds in several mags. Lock and load each mag and fire the rifle while holding it as loose as feasible. If the bolt doesn't lock open then take the GP up to the next size drill. You can do this on the range if you take along the proper tools. I've drilled (more correctly, reamed) GPs on the line using a cordless Dremel so it's not impossible.

"Each one is a little different. You'll have trial and error it. You don't want to go over 0.125" as that's the ID of the port in the FSB. It'll most likely be larger than .104 if you're using a .75" diameter FSB seat. "

[b]One things for sure, that should make gas port erosion a NON-ISSUE[/b]

What was acceptable reliability for you?  I've heard other owners who were just extatic about these setups.

Acceptable reliability. 6,000 rounds with less than 5 malfs of any kind.