The magstrike is one of my all time favorite blasters. It is also one of my all time least favorite to open...solely because of the confounded orange ring.
What I do to open mine is use a dremel with a cutting bit, and carefully cut two slits on the orange ring. One under, and one as close as I can to the top.
You may be thinking that this doesn't sound like a clean way of doing it. However, the last time I did this, both halves of the cut orange ring stayed on the shell. I was able to open up the blaster just fine, but the halves of the orange rings stayed glued on their sides of the shell. So besides the two noticeable dremled cuts, the ring stays on the blaster and you can easily open it risk-free. Not supah clean, but it does get the job done.
I've tried prying it as you have with no success, even tried hitting this bad boy with a heat gun and using a pair of pliers: nothing. How that thing is secured on there, we may never know. But if you try the dremeling method, all you have to do is a bit of sanding to clean it up and it won't look too shabby. I personally think it's your best bet, as it's what has worked best for me.
Hope this helps!