The barrel for my AR build will be 18", have a mid length gas port and a low profile gas block .  I would like to use a FF tube just long enough to cover the gas block and attach an end cap if possible.  It is my understanding that the gas port on either a 16" or 18" barrel is located 9" from the chamber.  If I get a mid length FF tube, will it cover the gas block?  If not, will a specter length tube do the job or just go with rifle length and be done with it.

 

Any plans for a buis system on this?  If so, the longer the tube, the longer the sight radius, the better aim you get. If not any of the tubes you described will fit Only question is that of weight of tube.  Personally like the 12-15" tubes for better feel with the weak hand on the forearm.  For reference see Stag 's Model 3GH upper (15")

the 9 inch should just cover the gas block. might even have a half inch or so of the block stick out as the port should be 9 inches from the receiver. that would probably eliminate the need for the end cap.

Any plans for a buis system on this?  If so, the longer the tube, the longer the sight radius, the better aim you get. If not any of the tubes you described will fit Only question is that of weight of tube.  Personally like the 12-15" tubes for better feel with the weak hand on the forearm.  For reference see Stag 's Model 3GH upper (15")

 

Yes on the BUIS.  I will probably be using Palmetto's front and rear flip set.  One other thing that should be considered, and I'll have to look this up myself, is clearance.  I'm going with a medium contour Ranier or ICE barrel.

specter will as far as I know will not totally cover the gas block..

mid length definitely will not because mid length ends before a standard AR front sight on a mid length gas system gun... 

Yes on the BUIS.  I will probably be using Palmetto's front and rear flip set.  One other thing that should be considered, and I'll have to look this up myself, is clearance.  I'm going with a medium contour Ranier or ICE barrel.

 

I hear Ranier makes a good barrel.

Still think a 12" or longer is the way to go.  As far as inside clearance, do not know of any problems when using low profile gas block.  Smallest OD tube I know of is 1.75" (MI, I believe) and it fits as far as I know. Have a YHM TJ 12" on a 300BLK build, works real good, and a Samson 15" waiting to go on a 18" build that I am working on!

This may help:

Go as long as possible, even a 15" You will never complain of having too much handguard, but to little can be an issue.

Thanks all, especially High Exposure.  The chart really helped put my question into perspective.  Although I've seen just about every iteration of barrel length and hand guard, this is the first image where I've seen them all side by side.  Well, I guess it's settled then.  I'm going with a 12" tube.  I know longer may be better for open sights but I do not think that will be a problem.  Longer shots will be handled with a fixed low magnification scope.

 

My original plan was to mount a front flip sight on a gas block with top and bottom rails but was advised against it since I'm free floating the barrel.  I was going to get a tube that left the block exposed but when I changed from that plan to the new "cover the gas block" plan I just couldn't find an acceptable answer to the length issue until now.  Thanks again.

 

Now that this matter is cleared up I think I'll post a complete list of what I've spec'd out for this build.  See if anyone spots a problem with the configuration.