I am trying to install a sling adapter which requires removal of the castle nut on the lower. Thing is pinned in and wont budge. Did S&W intend for this not to be removed? Any thoughts on how to do this other than brute force? I have the special wrench for this...would hitting it with a hammer (the wrench not the gun) cause any damage to the gun itself? Thanks for any help you may have....

If it's pinned you need to remove the pin. Should be staked, not pinned.

-- Chuck

There is a special wrench for it and it (the castle nut) is staked so it doesn't come loose. Once you have the right tool you can "break" through the staking without much difficulty. I got mine from Rock River..

Google:
AR15 castle nut tool
AR15 stock removal tool