Opening Proof Set Experience.

I found a vise/chisel is the best way to open them and not make a complete mess.
I put the proof case in a vise I have in my workshop, and then I put a chisel on the seam and give it a nice "whack" with a hammer, but not too hard since I just want to "split" it open and not have the chisel actually go too far into the proof case and risk hitting the coins.
I then "rotate" the case in the vise 90 degrees and work my way around it, doing this on each side.
By the time I get back to the first side, it is usually falling apart and I can usually just remove it from the vise and pry it apart and get to the coins.
There definitely is a 'risk' that you can damage the coins if you hit the case too hard and the chisel goes through and hits the coins, but so far, haven't had this happen.
I am also thinking of trying my circular saw on it, but am worried about flying plastic or the chance of hitting a coin with the saw blade. But in theory, you could just saw off about 1/2 inch of each "side" of the case with a circular saw and be able to open it that way. May give that a try next time I have a set to open and see how that works, but may want to make some sort of "holder" to keep it in place against the saw rail.
Michael