I'd hate to think of how many of them I've salvaged lead and copper out of over the years as I used to splice copper cables. But I tried a new method awhile back and thought I'd pass it along as well as a couple other things to make things easier.
The smaller ones are, IMHO, opened the easiest by cutting them into 4-5 foot lengths. Then go to the middle of the piece and "ring" the cable with a knife. A couple of bends back and forth will make a clean break in the sheath and you can slide the lead sections off. Take the copper and tie a knot in it and you've got a small batch that you can sell to the scrap dealer too. This would work pretty well for up to about 100 pair cable. MAYBE, slightly larger too.
When you get much bigger than that the lead isn't going to slide off as easily. We used to use an angled "chipping" knife to open the bigger ones along with a small hammer. Trick is to angle the knife and slice through the sheath which isn't really that hard to control without getting into the copper pairs which we needed for splicing and now want to save for the scrap dealer. Took a lot of taps to do a piece of cable.
Enter the handy little air chisel. I probably cut the sections about 5-6 feet long for this operation. A side benefit is the air chisel not only eliminates all that hammering by hand but when doing it by hand one had better luck anchoring one end of the cable so it didn't slide around. The air chisel eliminates that part of the task. A water pump plier makes it easy to bend the lead edges back so the copper pairs can be extracted.
Todays score included a large splice sleeve which that chisel walked right through with no effort and even used it to cut the thing into small pieces for the pot. Wish I'd have tried that air chisel years ago. ALMOST, makes it fun!