I've built my step father a 16" barreled AR. I had the compensator blind pinned and welded, ground off the bayonet lug, and now comes the pinning of the M4 buttstock after finding out dad likes it best in the half way position. What is considered an acceptable way to do this to keep him legal? I'm in Georgia, so none of this applies to me but that is totally irrelevant. I'm not as astute in Jersey laws as you obviously are. But I'd hate for something stupid to happen to him because of a technicality I missed. Thank you very much gentlemen!

Position the stock, then drill a blind hole through the stock, and into the bottom "track" of the buffer tube (where the notches are for the adjustment).  By "blind pin", I mean a hole that doesn't protrude from the other side.  Bang in a roll pin, and the stock can no longer be "collapsed".

This might be a slightly different stock than you have, but it should give you the general idea.  Drill the blind hole at the red dot:



As long as the stock can't collapse, or requires tools to change it's length, it's considered "legal".  I recently bought a SIG716, sold as a "ban-state-compliant model".  It had it's stock extended out all the way.  By removing the screw, and flipping around a little metal "insert", you could reassemble the stock in a shorter position.

I used an 1/8" roll pin for mine and my son's rifle and it looked pretty good. I was able to hide his under the locking lever on a Magpul so it is not even visible unless you squeeze the lever to move the stock.
PS: Home Depot 2 pins in a little bag for less than $1.00

Mine have small roll pins installed as well.

Thanks, you guys are great. The blind pin adjustable stock job was almost as easy as installing a front sight post on an A2 FSB... I was just making sure that the AR could be pinned in the 3rd (half way) position instead of fully extended because the weapon had to have a certain length or something like that. Also I was making sure that the blind pin job would be enough to suffice the Barney Fife's of NJ. I really appreciate your help. Thanks!