Maybe my 5.56 with a 5.5" barrel that would keyhole at 50yds was the exception. I'm sure yours is sub moa out to 500yds...
From my personal experience I would say you are so wrong. So there...
Why are target rifles all in the 18"-24" range if barrel length has no impact on accuracy?
You are using the wrong bullet weight and or rifling twist.
They use longer barrels for velocity which minimizes other factors ( drop and wind). If you lock a 5.5" ar in a vise it'll be just about as accurate as a 16" barrel. Of course if you screw up and don't stabilize your bullet you'll have poor accuracy.