https://colfaxtactical.com/docs/AR-15%20manual.pdf
1. With the upper receiver attached to the lower receiver, and the pivot pin and takedown pins in place, perform the
following test:
(a) Apply hand pressure to push the upper receiver as far to one side as possible.
(b) Attempt to insert a 0.020 inch thickness gage between the pivot pin lugs of the upper and lower
receivers.
(c) If the thickness gage penetrates to the pivot pin at all accessible locations, repair by replacement of the
upper receiver (see (b) below) or replacement of rifle is required.
2. If the rifle fails the above test, remove the upper receiver and install a "NEW" upper receiver and perform the
test again.
3. If the rifle now passes the above test, it shall be considered serviceable and continue in use.
4. If the rifle falls the test with a new upper receiver, this failure shall be considered a shortcoming. This
shortcoming requires action to obtain a replacement rifle. Once a replacement has been received, evacuate the
original rifle to depot for overhaul.