I have registered a 16" Colt 6920 as an SBR. I would like to bring the barrel down to 10.5"
A barrel can pretty much always be cut, it’s just a matter of effort and/or cost.
As for the gas port, if it’s correct now, it will need tweaking. I cut a customers barrel the other week, went from 14.5" to 10.5", and it had been so overgassed it ran just fine, even with cheap wolf.
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I’d opt for a barrel swap if it was me. G&R had some Colt 10.3" barrels in stock; not sure if they still are.
If you decide to cut the barrel down, the port will have to be modified.
I’m not sure if it’s cost effective to cut one down vs buying a 10.3" & selling the 16".
For a 10.5", I would actually prefer to start with what you have because you can specify your gas port instead of living with what you buy.
Bingo.
Send it to adco. They can port it to your specs.
I’ve done it, BTW. not a baseless plug.
I’d rather have a factory commando complete upper. Sumuse’s way def works though.
Since the OP asked if the GP has to be altered, I’m guessing he may not know what his “specs” are. 
Did Colt get it wrong on their factory 10.3 barrels?
Curious what Adco charges for cut, thread, crown, & modify gas port. Looked like it’s close to $200 but I only scanned over their prices.
I don’t see it being cheaper than buying a Colt 10.3 & selling the Colt 16".
Don’t get me wrong, I know many people have done it. There’s also people that may have special needs so that route makes more sense. I’m just not sure that route would be in the best interest of the OP.
I had a 6920 barrel cut to 10.5 with no gas port work done. I am also running an adjustable gas block and suppressed about 95% of the time. No issues so far with about 1k rounds.
That’s very interesting. The gas port on a Colt 6920 is right at .063. Are you running it suppressed all the time? Considering that in testing they had to open it up to .071 to make it work.
I had specified they not make any changes when I sent it off. Only problem is I failed to measure the port once returned to confirm they didn’t. Suppressed most of the time, only time it’s not is when people want see how loud it is with the SF brake.
If it helps other parts included is H3 buffer and LMT enhanced carrier.
Will be shooting unsuppressed the majority of the time.
I agree it may just be easier to get a factory commando upper. Thanks for the responses guys.
The LMT carrier and the H3 buffer in theory should actually make it choke, especially if it has the standard port size.
It is interesting. I did the same thing with a 16" DD barrel and it did not function unsuppressed with a million different types of ammo.
Well hell if I had my tools I would tear it down and measure it. Unfortunately I moved at the beginning of the year and everything is in storage. Primary ammo has been xm193 and m855 with a mix of mk318 mod0 and federal LE223t1 with no issues.
I really do not like the narrower band of function in 10.5" 5.56 carbines, just adding an inch of barrel in those increases function by a wider margin. In my opinion, a 10"ish carbine gassed carbine based action is better suited to specific applications that are narrower than what most would expect.
For many reasons, I would tend to recommend 300 Blackout for barrels under 11.5" in operation. You can make 5.56 run in shorter OAL setups, but how wide is the operating range in function?
You scanned too quickly. It’s $65.