IIRC, a lot of MS-Contins have a max matrix inside the pills so if you break the 100mg in half you'll still have a 50mg MS-Contin. As far as crushing them up and snorting them goes, good luck if they have the wax matrix, some have had success microwaving or using other methods to make the morphine instant release, when I have the luxury of having morphine (and the inconvenience of it being MS-Contin) I usually just chew the pills, or crush them up and then stir in a solution of high proof ethanol and water in a small container (think shot glass), using vodka when everclear isn't on hand, and then drinking the mixture.
To each his own, there are (or were) quite a few excellent threads on here about the various MS-Contins available. Do you know what brand yours are?
Edit: after reading your post again, it sounds like you have the same ones I had, the Mallinckrodt generic, those should be (again, IIRC) the ones with the wax matrix. I'm not familiar with the 100mg ones you've described.