What weight buffer are you running?
5.4 ounce, as supplied by Colt
Gotcha. You can probably go heavier. I had a cheap source on ebay that I didn’t know was going to dry up. 
I have been running 6.1’s in my x39 AR’s. They were $5-6 shipped… I have some .308 springs to try for the x39’s too. I wonder if they would function in a 9mm?
Since it runs 100% with Q4318, I’ll leave it stock.
UniVibe, that’s pretty much where I am…if it ain’t broke, don’t fook with it. 
Moon