First and foremost, I do understand that I need a permanently attached device, but I am trying to make my 6920 as close to a 6921 as realistically possible and not damage the value of my Colt. My options appear to be:
Barrel replacement option: where can I buy a 6921 barrel with a Vortex permanently attached (are there better options than the Vortex, given that the A2 is too short NFA-wise for a 14.5" barrel)?
Barrel cutting option: if I get my barrel cut down to 14.7" (to use the stock A2) or 14.5" (to use a Vortex or other), will my barrel be otherwise identical to the stock 6921 14.5" barrel? (i.e., are the M203 cut and the FSP mount in the same place distance-wise from the upper on both the 6920 and 6921 barrels, and would I need to re-zero?)
Upper swap option: where can I buy (or exchange my unfired 6920 upper for) a Colt 6921 upper with a permanently attached Vortex?
Sabre Defence makes an extended A2 flash hider that when permanently attached makes a 14.5" barrel legal length. I would recommend not cutting on your barrel or permanently altering it if you are concerned about value. That being said, on my guns if I know I will never sell it or get rid of it I do what I want to make it the way I want/need it. YMMV.
I’ll be the oddball here. Just left a barrel to be turned and cut down. Unknown mfg. HBAR that shoots OK. Not chrome lined. I wanted to get the weight off and thought I might as well get it shortened at the same time. Why? Just because I wanted to and had it off anyway. Now or never. $90 total. I wouldn’t have been “happy” unless I did it…
Why would it be against the law to chop a Colt barrel after it’s left the factory? As long as someone qualified does the work and you end up with 16" overall with the perm. flashhider what’s the problem?
I’m not saying it’s worth it … but why would it be against the law?
A 14.5" Colt M4 barrel with a permanently attached Vortex flash suppressor will be approximately 1.1" shorter than a 16" Colt barrel with an A2 flash suppressor.
Cutting a 16" 6920 barrel to 14.5" will not give you an “otherwise identical” barrel to a stock 14.5" 6921 barrel as the barrel stamps are positioned differently. While the dimensions will be the same, you will have destroyed most of the original barrel stamp in the process. You mentioned your concern for not damaging the “value” of your Colt; well in the eyes of many true Colt aficionados, this will have made the barrel next to worthless (as evidenced by many of the previously posted comments.)
M4
Your best bet is to try to find a 6921 barrel (or better yet, an M4A1 barrel [6921HB]) and have a Vortex flash suppressor permanently attached and then swap out the barrels.
I never understood the point of sub-16" barrels with a permanently attached FS to bring it to legal length. You get all of the negatives of an SBR with none of the advantages.
This applies less to 14.5" barrels than 10.5" barrels with permanent flash hiders.
Just an observation, but is the gas port in the barrel a different size for 16" versus 14.5"? If it is, a shortened barrel could create reliability or function problems.
