Here's a little tip I picked up lately from Mr. Tippie.
It sounds like you don't have a problem - but rather a blessing! As you are most likely aware, many AR's (including mine before my Accu-wedge) tend to "rock" a little bit in the receiver. Chances are, if your rear takedown pins are loose and easily removeable, you have a good bit of rock. A tight fitting receiver will be VERY HARD to remove the pin - anyway from having to push extremely hard with your thumb, to punching the thing out with a hammer and chisel. The latter, although more annoying, is the most desirable.
When you have that rock, your upper receiver (based on the lower as an axis point) can move both up and down AND cant left/right. During your shots, you're only securing the lower receiver...so the upper is free to move in odd directions that you can't account for. Now, for carbines this difference is negligible in plinking and possibly short range target. But for 300 yards, you'd have a better chance of scoring a prairie dog kill with your ironsights incorporating a nice, stiff receiver.
[img]www.auburn.edu/~littlcb/new jew.jpg[/img]