I am looking to put a 16 inch barrel on an A3 upper and would like to have: full length sight radius using flip up BUIS (scope will be primary), mid length gas, chrome lined, 1:9, and 5.56. I plan to have BUIS so I don't need the larger front sight on the barrel.  Do I buy a stripped barrel or is it more cost effective to remove the original front sight if I buy a dissy?  Are dissies carbine length gas and not mid length? How do I mount front BUIS if I do get a stripped barrel?
Do these specifics limit me as to which companies I can get a barrel from?  Do DPMS or Bushy dissies have these options? What is my best bet for this barrel?  Thank you in advance for your answers.

Ok so you want both front and rear flip sights correct? You can either get a free float quad rail to mount you front sight onto, or get a gas block with a picitany rail to mount the front sight, or get a gas block with a folding front sight built in. Sounds like you would be better served with the rifle length free float quad rail since you want the longer sight radius and a flip sight. Get a barrel that is set up for the midlength gas system and get a low profile gas block and midlength gas tube, flattop upper and rear flip sight and get a flash hider and bolt carier assembly and charging handle and you should be good for your upper. Oh yeah and you can get this built by someone else i'm sure if you aren't up to it. I like CMMG a lot and would check them out.

If you want to use a flip front sight, you can use any 16" mid-length barrel and the Yankee Hill clamp-on flip front sight/gas block.  Because the barrel in front of the "real" gas block is usually smaller than .750, not all gas blocks/front sights work in the "secondary" rifle position.  They YHM clamping front sight works like a scope ring and therefore compensates more for this variance.  

Even Bushmaster's Dissipator is .740 in front of the GB and uses a special front sight with a slightly smaller diameter hole.  However, since the M4 profile is slightly larger than an HBAR in front of the gas block, you're still more likely to have success with other gas block front sights than a standard HBAR or A2 profile barrel. (.730 I believe)

CMMG medcon barrels have the .750 step in the rifle position so you could use any clamp on front sight/gas block, but they are 1:7 twist.  They are currently the only place I know of making these as a regular item.

JTAC did make a barrel that had the correct .750 step and was 1:9, (ESR Barrel) but I think he discontinued them.  I know I bought one on sale for $200, and they're stainless steel, Mike Rock(maybe?) barrels with a wylde chamber and are very accurate.

ADCO also offered the "dissipator style" barrels once upon a time, but I think he discontinued them as well.

So tell me if this is correct...My options seem to be:

Stripped DPMS Dissipator (with Chrome lining option added) which, I think, has the Rifle length gas system....

Stripped Bushmaster Dissipator (with Chrome lining option added) which, I think, has the Carbine length gas system.....

Any 16" mid length (chrome lined), put on low pro gas block, and then put a clamp on flip front sight where a dissy sight would normally go....

Or should I not even worry about a front flip site, and use the regular front irons?  Am I going to run in to problems doing that with a scope attached to the A3?  Will the front irons be in the scope picture?

If I decide to go with the DPMS or the Bushy, which is more reliable, Carbine length gas or Rifle length gas on a 16"?  

Thoughts?

I believe a standard front sight will give you problems w/ a telescopic sight, w/ a dot sight (low to no magnification) it probably won't be an issue. Personally I think the standard 16" bbl w/ mid-length gas, lo-pro gas block and rifle length rail was the best option mentioned above, but the free float rails aren't cheap. The bushmaster w/ carbine gas is a good option as well.

height=8

Quoted: Sounds like you would be better served with the rifle length free float quad rail since you want the longer sight radius and a flip sight. Get a barrel that is set up for the midlength gas system and get a low profile gas block and midlength gas tube, flattop upper and rear flip sight and get a flash hider and bolt carier assembly and charging handle and you should be good for your upper.

I'm thinking that this might be my best bet.  Middy and low pro gas block with rifle length quad rail with flip up front sight on the very end should almost give me my rifle length sight radius, correct?

height=8

Quoted: I believe a standard front sight will give you problems w/ a telescopic sight, w/ a dot sight (low to no magnification) it probably won't be an issue. Personally I think the standard 16" bbl w/ mid-length gas, lo-pro gas block and rifle length rail was the best option mentioned above, but the free float rails aren't cheap. The bushmaster w/ carbine gas is a good option as well.

Thanks a lot.  I think I was writing that one post while you were answering.  I appreciate the input a lot.