If you have a barrel of fast enough twist to stabilize a 200gr bullet of that caliber (must be a custom job as I'm not aware of any factory rifle with that fast of a twist), 1800 fps would make it a deer rifle to reasonable ranges. In general most 36cal rifles are twisted for round balls, and while I'm sure they could kill a deer I would not call it a real ethical combination.

Check your state regs, both states I've lived and hunted in have regs that would make what you are planning illegal for deer.