My form 1 stamp just got approved on my Quarter Circle 10 9mm colt style lower so I'm gonna start getting some parts together.
I've ordered a 5.5in barrel but I'm second guessing myself, thinking I should've gone with something longer since this might limit my rail/hand guard options. What are some good rails for this short of s barrel?  FYI....I don't currently have plans to run a suppressor on this gun at the moment.  

Next question...
What upper?  
I kinda like the Gibbz side charger.   Or should I just keep with QC 10 parts?   Will side charger uppers have compatibility issues with my QC 10 lower and bolt?

If you are not running a suppressor, then I'd pick a barrel length that maximizes ballistics.  When I was looking at the tables an 8-11 inch barrel range was optimal in my eyes.  In length vs ballistics.  

In that range of barrel lengths I'd pick a hand guard you like and then order the barrel.  Instead of the other way around.  

I have a 3 inch barrel with a pretty much permanent suppressor attached, an 8.5 inch barrel and a 10.5 inch barrel.  They all balance well.  

I looked at the tables at ballistics by the inch.  I did NOT do any testing on my own.  I relied on the data at that website.

Well I figured at 5 inches, I'm about at the same ballistics as any full size handgun.  I also hear that I'm not gonna get any accuracy gains with anything longer.  Might be nice to have a slight velocity gain with a longer barrel.  
I like your idea a of getting the rail first and picking the barrel to match it.   I screwed that up for this build since I wanted to take advantage of qc10's 20% off sale.

Thanks!   I'll definitely keep you in mind!