I am loving my M&P 15. But the last two times cleaning it after being at the range, I have found my buffer retainer and the small spring below out of position. It has been in my lower receiver near the trigger assembly. Full disclosure time, I did recently install a MAGPUL ASAP (Ambidextrous Sling Attachment Point). As part of this install, I had to remove the buffer tube, spring, and castle nut. And yes, the buffer retainer issue only started after I installed the ASAP. Sooooo, what did I screw up and how do I keep the retainer from popping out again?

I'm assuming the following:

1. On my 15-Sport, the castle nut is staked to the end plate. If yours was staked in a similar fashion, I assume you had to peen back the staking or grind back the staking to get the castle nut to move.

2. When you reinstalled the buffer tube, you correctly positioned the lip of the tube to retain the buffer retaining pin.

3. Peening back or grinding the staking can ruin the castle nut(?). If it did, you replaced the castle nut.

4. You used a castle nut wrench to tighten things down.

5. When it was all tightened down and in place, you used a screwdriver & hammer to stake the end plate & castle nut in place to keep them from vibrating loose.

The alternate is to use a dab of blue thread lock on the buffer tube. Just remember that to break it free later will require a little heat.

Now if all of that was done, then inspect the lip of the buffer tube that is supposed to retain the pin. Is it chipped or damaged?

Just give it an extra turn so the buffer tube is tight against the shoulder of the retainer.
Then make sure the castle nut is tight.
With the way the latch plate works you really have no need to stake the castle nut if everything is tight.

__________________
Hold my beer and watch this!

Thanks JaPes and Gunslinger for the ideas. I took my lower receiver to my local gun store and they pointed out the issue. When I served in the Marines, I carried the M16 A2 with a fixed stock. My M&P has a collapsible stock. I had to back out the stock to put in the Magpul ASAP. What I didn't realize is that the threads from the stock actually holds the buffer retainer in place (as Gunslinger pointed out). $1.50 later I had a new buffer retainer spring and was able to fix the issue.

Thanks for your helpful information.

Well now you know.

And you know what they say...