I had the buffer tube castlenut loosen on me.  How do I properly stake this?  Tools, method etc.

A center punch, or even a nail set will work just fine.  If buying a center punch, make sure not to buy the "automatic" kind as they are spring loaded.

I only use my AR15 at the range, so I just bring my castle tool, and tighten up before and after the session.

This Will Do

I put a drop of the weakest locktite and used the ar15 tool to tighten it down. Hasn't come loose.










 

Yup

Auto center punch worked pretty well for me. Takes about 10 strikes (no hammer needed) and it's done.

I'm curious why you say not to buy the automatic kind. Auto punches like this make staking castlenuts stupid simple.

Scroll half way down this page for pics of a castlenut staked with an auto punch.



First, make sure to tighten the castlenut down well with a castlenut wrench. Then place the punch (which ever one you use) on the lip of the endplate next to the little cut out notch on the castlenut. You only need about 1/32" of material on a standard endplate. Then just give the punch a firm whack. It may take a couple hits. One of the reasons I like the auto punch, is that you can hold the lower in one hand (or down on a towel on a work surface) and the auto punch in the other. No slipping punches and no whacking. And it moves the material gradually and consistently. It's virtually fool proof.

ETA: cmcflex beat me to it with the pics. His is a better representation as it is a standard endplate. Mine was on an ASAP, which requires you to move a little more material.

Properly doing it uses a big ass press in a jig. I just use a center punch as stated above.

An automatic center punch is my preferred method as well.