Greg -
Today I'll start swapping things around and post.
On another forum, I came across a thread where someone was rather disappointed with the end result of his 20" custom varmint AR. This guy has the experience, did the research, and everything looked good on paper. End result was that is velocities were way slower than they should have been by the math on paper and accuracy was going to suffer due to the slower velocities.
This was then confirmed by several others that went through the same thing. Consensus - there is little if any appreciable gains in the extra 4" of barrel. Further searching for research and found tests by other agencies to confirm his results.
He was expecting 160-200 FPS more velocity over his 16", and only got 80, and in one case, the 16 was actually faster.
It would seem that the theory of a longer barrel on a gas operated system really does not give you any appreciable advantage over a 16".
Here is a link to the thread. You might want to factor the information into your barrel decision.
http://www.defensivecarry.com/forum/defensive-rifles-shotgun-discussion/181455-ar-223-556-longer-barrel-may-not-do-anything.html
After reading that....I began to wonder about my Mk12. I hope I will not be disappointed for the extra money I spent for that. Bad enough as it is that I could have picked up an AR10 for as much, just the cost of reloading for that goes up significantly.
I wish I could have given you a better answer on the accuracy of the Colt upper. I've not shot one, all I know is from what others have reported and 2MOA or better is what is to be expected (given most shooters are using factory ammo, not reloads), and they are being put to use by and large 100y or less.
I have zero experience shooting any other ARs than my own for accuracy. I've shot several others with the goal of placing my shots inside the 5"x3" rectangle, or keeping that suppressed full auto on the target, even grouping somewhat rather than a string.
Not that many people buy a civilian version of the M4 and plan to go tack driving with it. It's people like us that load our own ammo to suit the rifle, and have the skills ~ however rudimentary ~ to shoot accurately at range.
You pick the projectile to suit the twist rate and barrel length, use a powder with the proper burn rate for the barrel length and gas system, and adjust accordingly. This may end up being a rather tedious endeavor once you get the calculated projectile RPMs figured if you are over spinning or not spinning fast enough to have optimal stabilization. You may have to change projectile, powder, or both.
My Mk12 barrel was designed specifically for the 77gr Mk262 round. Somebody else did the work for me. I have the projectile that it was designed for, I now just have to pick the right powder and the right charge to get optimum performance because I am not buying Black Hills ammo for 5x what it will cost me to load it myself, and the powder they use is proprietary info. Powder burn rate for barrel length is something that I have not delved into even remotely. I am using the powder that a local gunsmith uses for the reloads that he sells.
I got lucky, plain and simple, with my 16" and am able to get 1MOA out of it using my reloads. If I had the time and money to try different powders I might get it to do sub MOA. My goal with the MK12
is to get sub MOA, and at most 5" at 600y on demand. First I need a proper optic. Then I can start to play with my loads (after I get the time, money, and a place to test).