This is a silly question.
Why not get a new upper with mid-length gas and a long rail?
You’re handicapping yourself sticking with an old school Colt upper.
Ive got a 12" C4 Mlok fsb cutout on my 6921 upper. Its rock solid like the OG C4 but slimline. Nothing wrong with a carbine gas system.
This is a silly response that doesn’t answer the question and shows a level of ignornace. There is abosolutely nothing wrong or handicapping about sticking with the Colt upper.
OP rails are personal preference for the most part. I prefer thinner, lighter and longer with only as much PIC as necessary. Others prefer larger PIC laden rails that are bomb proof. You have a lot of good options above and it sounds like you are zeroing in on your choice.
From the Similar AR Setups thread…
By your own words you don’t train much, and it’s evident in your recent posts. He’s not handicapping himself by using a carbine length gas system, but you sure are by not training and shooting enough to know that.
With a BFA? I’d love to know the story on that pic.
The centurion is a great rail. I have one. If the bottom rail wasn’t so tall, I’d have those on a lot of guns.
Force on Force Light, now with less budgetary calories, and a less filling risk assessment while still tasting great.
I didn’t even know you there was a mk18 BFA. I have a whole bag of blanks and an old Bushmaster 20" upper with a BFA. If the powder supply doesn’t improve, I’m switching to that set up. 
If you’re going with a RAS, the Free Float Ras is probably my favorite quad rail of all time. 
Great, light weight and strong (it’s no RIS II), while free floating.
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I’ve never had the chance to try a FF RAS. (got the old Wilcox mount)
Yep, couldn’t agree more. I’d still buy complete carbine length rifles and uppers if the price was right. However, all future carbine builds will most likely be mid lengths.
NYH1.
Im a carbine fan, it just works! i’ve got ml rigs but love me cl length and love the ras with fsb barrels
If you’re looking to just mount a VFG and light, I recommend you go with KAC M4 RAS. Super simple, solid rail, doesn’t add much weight to the rifle.
There is nothing wrong with a Colt with a carbine length gas system, unless the barrel is shot out there is no reason to ditch it.
I didn’t see it mentioned but the OP needs to consider the level of work and knowledge/tools required. There are basically three levels…
- Simple drop-in like style the KAC RAS or Magpul SL
- Handguard that uses stock GI barrel nut but requires additional work such as the several times mentioned Centurion C4 series.
- Complete barrel nut replacement.
Personally, I’d go with a Centurion Mlok C4 with a FSP cutout and cut off the delta ring assembly, forward handguard cap and sling tab.
Is BCM discontinuing the QRF?
I would hope not. What makes you ask that? I did notice many of the lengths were out of stock a few weeks ago.
The MOE-SL polymer handguard is the clear and distinct winner for drop in carbine length handguards.
The KAC RAS is an awful relic of the past, please don’t waste your money on that; if you’re making a clone rifle or something, ok, but otherwise, it would serve no purpose besides making your rifle heavier and less ergonomic.
I have never reported a post faster.
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For a polymer handguard, I prefer the BCM PMCR over the MOE SL after using them both. The BCM is an eighth of a pound lighter and surprisingly rigid for a polymer handguard.