I'm not expert, but I had a client that had some problems with breaking into those types of things, and research into building caskets for a story, I can give you some ideas.
First off, geography makes a difference.
In the northeast, cemeteries have a temporary structure for holding bodies until spring. Digging a hole in winter is a pain in the butt around here. This structure will be underground, normally set in a hill, and secure by a heavy metal door, with a lock. We also have mausoleums, although not very many. My understanding the doors are steel or iron, and secured with a lock. Keys are maintained at the cemetery's office.
My client used a wrecking bar and sledge. Reasonably effective, but rather noisy. Which was how he managed to have the misfortune of meeting me. Seems the pounding woke up the neighbors who called the guys with the blue light special. He was surprised to find out disturbing the dead is a felony in this state.
Talking to the cemetery people, their preferred entry method, as the locks can be frequently frozen shut, is a cutting torch. Sometimes you just have to take out the whole door, but a lot easier on the back. And probably disturbs the neighbors less, unless your character wants to see how pretty blue strobes look over a misty cemetery.
In the northeast, we have a moisture problem and wood caskets will be exhibiting various stages of rot. Older ones are probably easier to open with a sledge, if you're not worried about being neat. Caskets tend to use more fastenings than are structurally needed.
N.C.I.S. has an episode where the killer is storing bodies in a large mausoleum in the individual tombs. These were sealed merely by unscrewing the face plate.
Best of luck,
Jim Clark-Dawe
