Receiver slop is the common name for the amount of movement between the upper and lower receiver of the AR-15/AR-10 weapons platform. Although it is commonly accused of causing functional or accuracy issues, it is in reality just an annoyance and side effect of mating Mil-Spec parts. No two AR-15 parts are ever exactly the same. Mil-Spec is the set of military specification governing the tolerance or amount +/- that a part may be from ideal, while still being a good part. Most weapons manufacturers actually hold their weapons and parts to a much higher standard than true Mil-Spec. For example, a true military spec weapon may be missing up to 20% of its receivers' anodizing.
Receiver slop is measured by using a feeler gauge to test the width between the pivot pin lugs on the upper and lower receiver. To take the measurement, you twist the upper and lower apart to force the opening to be as big as possible, and then slide a feeler gauge into the opening all the way down to the pivot pin, on both sides.
In order to fail, or be too sloppy, you must be able to fit a .020 feeler gauge all the way to the pivot pin on both sides. At that point it is not within Mil-Spec, and either the upper or lower needs to be replaced. The feeler gauge pictured is a .019, and will not even begin to fit inside of the opening. The sloppiest set of receivers I have ever had would not even fit a .011 gauge, which was the smallest siguage ze that I had available.
There are several cheap and quick fixes to receiver slop, when it is not enough to actually be out of spec. The easiest would be the Accu-Wedge, which is a small rubber bushing that sits inside the lower receiver, under the takedown pin lugs. This forces the receiver tight, and can be had for about $5. The other is to use shims or a small O-ring around the Pivot pin lug of the upper receiver.
If you have any questions about receiver slop or other related issues, please contact us at firstname.lastname@example.org and we will be happy to assist you.