if its cut / welded / modified so it no longer functions then it can be classed as a "deWat" and is no longer a fire arm (aka wall hangers)
if it still funtions, it remains a fire arm
now if your talking about "oh iv done XYZ to my rem 700 action (mauser, lee enfeild, etc and so on) as long as that serial number is assiged to that gun (useless with out LRG, but for restricteds ) it will always be classed as that make and model from a legal stand point
so no you couldnt take an AR, weld the upper / lower together, mill out a bolt removal port, modify the trigger group to drop out, and any other billion things one could do, then have it classed as NR