This is perplexing. I would tend toward believing it's a fit issue, but I have shot my two ARs with the stock fully extended, fully retracted, and pretty much everything in between. The recoil is not much more than that of a 22LR. The muzzle blast is jarring, though. Especially to those standing nearby. I don't really notice it when I'm shooting - only when some one else is.

I'm guessing something is amiss in your buffer setup or the stock you are using just hits you in the wrong place. Take the buffer out and clean the tube and spring. See if anything is broken. Maybe try a slightly heavier or lighter buffer or a newer spring and see what that does.