Most if not all collapsible stocks other than Colt originals have the end of the buffer tube cut flat. This is incorrect.  The proper Colt stock has a timing lip on it where the portion that is over the buffer retainer sticks out just a bit more than the top half of the tube does.

Screw your buffer tube in far enough that it will hold the buffer retainer down but that one more full turn is too far.  You'll find that you will not be able to close the upper receiver because the buffer tube is sticking out too far. File or grind off the upper partion of the buffer tube that is blocking the upper from closing properly and you will be good to go.  You will have just created the timing lip that comes on Colt telestocks.

If you do not have enough of the buffer tube holding the retainer down it will get battered.  This will enlarge the hole it sits in, ruin the retaing pin, and give you all sorts of problems.

Another AR15.com fix!