Here's the punchline: I have a complete Bushmaster lower that I bought in 2002 or so, and it's been sitting in the safe ever since.  I thought I would change out the buttstock from the collapsible "looking" stock...it's a non-collapsible from the dark days.  

I have a lower receiver block, and a spanner wrench...and that castle nut is not effing moving.  WTF???  It's as if they used nuclear strength Loctite.  I actually broke the tooth on my first spanner wrench.  Is there anything special that I might be overlooking?  

Yes, I'm trying to loosen counter clockwise.  

I cannot see that it's staked.

Hell, I'm wondering if I need to heat the nut up a bit with a Bernz-O-Matic.

No way should it be this hard.

There is a chance threadlocker was used, apply some heat and see if that yields results.

They probably used Red Loc-tite on it instead of staking it, (dumb)... I would take out the buffer and the spring, then heat the receiver extension up near the castle nut slowly for a bit and see if that allows it to budge for starters.

You can use a cheater bar for more leaverage.

I also think you should use a bit of heat after removing the spring and buffer.

I have a Bushmaster from 2010 that came with the same kind of stock. That castle nut was so tight on there I thought I was going the break the lower in a mag block. They dont use loctite but It took me forever to get the thing loose

HammerHead Rifle Tool with a breaker bar removes all castle nuts with ease.
http://hammerheadrifletool.com/HammerHead_Rifle_Tool/HammerHead_Rifle_Tool_Home.html

Topic Moved

Make sure you have a good wrench.  There are some cheap ones out there that only engage one notch on the nut.  Get one that engages at least 3 of the notches.  I use a RRA wrench, see link below.  If the nut is on there tight I hold the wrench firmly onto the nut then whack the end with a small hammer.  Haven't had a nut yet that didn't come loose using this method, including staked nuts.  I use a receiver block in a vise to hold the lower while removing / installing the nut.  

http://www.brownells.com/gunsmith-tools-supplies/rifle-tools/wrenches/ar-15-m4-stock-wrench-prod16882.aspx

Done.  A little heat fixed things up in a jiffy.  POS "tele-style" stock is a shitty stock/buffer tube all-in-one...so I need a new buffer tube.  Regardless, I got it.  Thanks.