So I'm building my first upper. I took advantage of  LaRue's barrel sale this summer and snagged a stealth and a predaTar barrel.
I'm wanting to make good use of the barrels and would like to build as accurate a rifle as possible.
I'm looking a using a BCM upper, they claim the fit is a little tighter and thus improved accuracy.
Looking at MI SS gen 2 free float handgaurd
Considering Centurion bolt carrier group.
BCM gunfighter charging handle and probably a phantom flash hider

Questions are what will play the biggest role in accuracy after the barrel?  What things do I need to be considering? Is there anything about my choices I could choose better? Best block to use to assemble the upper, I see the one in the ARFCOM store and I've seen the clam shell kind, is one better than the other?  

Thanks

Only other thing that matters is ammo and the guy pulling the trigger.

While I tend to agree that the barrel and ammo (you have to shoot good ammo as it is the limiting factor in your typical AR), if you wanted to go all out as an exercise in OCDism you could look at getting:

1.  A "better" upper like Vltor's MUR.
2.  A competition Geissele trigger.
3.  A Smith Vortex flash hider; rumored to be "accuracy enhancing", if nothing else it is one of the best flash hiders.

After that, it really will be nearly 100% you.

I know ammo matters and will be buying good for zeroing, testing and working on improving my skill.  One day I'll get into reloading.  Its taken me a long time to get to be able ti justify the parts for the builds.  I've had the lowers since 2005.  Kids in private schools eats up a lot of fun money, chosen sacrifices, oh well. Hopefully I can get to the point to truly notice differences.  This will be my first optics AR. My go to rifle for the past 11 years has been an A2 upper Bushmaster purchased just after the sunset.  I have thought hard about the Vltor upper but not quite sure if I would be better served by the extra cost or using that money to finish another upper.  After the reminder I have heard good things about the vortex, not about the accuracy claims but enough I may consider that instead.  

Anybody with info on the holding blocks?

I'm excited like a kid in a candy store 10 plus years of waiting and its about to happen.

ETA: trigger may be a future upgrade lower has been built for a while I've been using it with the bushy upper.  Before a trigger upgrade I best finish what else I've got started

Ammo is a given.
The bolt can make a noticeable difference.
Depending on the shooter the trigger.

If your barrel nut has to be aligned the Wheeler clamshell insert makes that easy.

Read this LINK then you can make your own decision, I have a clamshell and a reaction rod, when I asssembled my upper the clamshell interfered with the barrel nut wrench I have so I ended up using the rod.
Some people lap their uppers for increased accuracy. There are a couple of older threads on this topic with alot of differing opinions. You should be able to search this forum and also the Precision rifles forum for the threads.