I sent the 16" upper off to ADCO, it ran fine. They found no problems.
I have new buffers and springs from White Oak.
As soon as I put that 16" on my RRA lower, I get 40-50 FTE’s out of 100 trigger pulls. I have a commercial 6 pos collapsable stock. As I said, if I put my 20"ers on the RRA lower, they function fine. My 16" runs perfect on my other lowers as well.
The only thing I can think of are your buffer springs, your buffer weight or your mag springs. How does your collapsible stock lower buffer weight and spring compare to your other lowers? Is it lighter or heavier? What is your spring type, and extra power or a standard? Whichever it is, swap parts out and head in the more favorable direction.
Regarding the mags, if your collapsible is cycling too quickly, your mags may not be able to keep up.
I can see your 20" running OK since it’s running lower gas pressure. It may run slow enough that everything’s OK, but when you toss in the carbine length gas system and the longish dwell provided by the 16" barrel, these are the areas I’d look at.
