Not reading through the volumes of stuff at this point, and simply going on the context of the recent posts I would point out a couple things...
"Spring guns" refers to guns operated by a spring (such as most air rifles) and is a really poor choice of terms if used to mean a gun set up as a booby trap.
Setting up a gun as a booby trap is mentioned in older laws (including some Fed ones, IIRC) and the term used then was "set gun".
The reasoning "set guns" are illegal is simple, and has to do with the employment of deadly force. Setting up a booby trap to protect your PROPERTY simply doesn't meet legal requirement for justifiable use of deadly force.
Doesn't matter if it is a gun, a grenade, a crossbow, a deadfall, or another device, if you are NOT THERE, you CANNOT judge if deadly force is justified, and if you are not there, to be at risk, and to make the judgement, then under the law, use of deadly force is NOT justified, or justifiable, and YOU are liable for any death or injury that results.
And, putting up a sign isn't enough to CYA, either. Nor does it matter if someone has to physically break through a barrier to get to the place where the trap is. You WILL be found liable and guilty for creating the situation and ALL its possible consequences.