I stumbled across some listings for Winchester Deer Season XP ammo in 450 Bushmaster. It appears to be a "250gr Poly Tip" projectile. I can not find it on Winchester's website and the stock photos show a bottleneck cartridge. I'd be interested to see if they sell the bullets as components separately in the future. Probably a really long ways off.
http://www.centerfireguns.com/winchester-deer-450-bush-250-xp-20-10-x450ds.html
https://grabagun.com/winchester-deer-season-450bush-250gr-20-rounds.html
https://www.outdoorgunsales.com/products/winchester-deer-season-020892224520