As Brent said, measure your rate of twist. How much does the rifle weigh? With your rifle being a .40 cal I would think a winged bullet but it could be a belted ball gun. I have a 2 groove 14 bore belted ball rifle by William Moore with an incredibly fast rate of twist, yet it shoots those belted balls wonderfully. If you go belted ball, do a cerro-safe casting of the muzzle and have Jeff Tanner make you a mould. Felt wads are very useful for creating a gas seal as well.
A great place to start if you have not read it already would be the Forsyth book:
https://www.amazon.com/Sporting-Rifle-Its-Projectiles/dp/1437339433 Steve
