How in the gotdamn do you open NAMCOT cartridges?

[kryptonaut]

Awhile back I got a copy of Megami Tensei. Works great, but I can tell it's seen heavy use. I want to open it up and try to clean it up a bit, but the casing won't bulge at all!

I'm doing the clamp method everyone's used to, but I swear NAMCOT infused their plastic with adamantium because all that's happening is my clamp is bulging instead of the casing .

I tried opening a different NAMCOT game I have, same result. I don't want to accidentally break my games, so I was wondering if anyone here has experience with this?