Legislation, as it often does, pushed innovation on that platform.

Via Guns.com:

The .450 Bushmaster caliber quietly crept back into the limelight this year, its resurgence linked to legislation from the Midwest.

How did updated hunting regulations breathe new life into this chambering?

The Thumper round, as it is affectionately known, was created by Col. Jeff Cooper. Cooper, notorious for his disdain for the .223 caliber AR-15 platforms, envisioned a cartridge for big game that could handle shots out to 250 yards.

Cooper’s idea led Tim LeGendre of LeMay Firearms to develop the “.45 Professional Cartridge” utilizing .452 projectiles. LeGendre eventually licensed the design to Bushmaster who approached Hornady with another idea.

Bushmaster asked the Hornady team to modify the original concept to integrate a .250 grain super shock tip into the design. The modification forced the ammo maker to shorten the cartridge case and thus the .450 Bushmaster round was born...

...Though the round has existed in shooting circles, it’s seen a resurgence in recent times in response to Midwestern hunting legislation. In states like Michigan, hunters are restricted in the types of cartridges that may be used while hunting with rifles. Shooters are limited to using straight-walled cases between 1.16 inches and 1.8 inches and of .35 caliber or larger.

Necessity, as they say, is the mother of invention.

While the Midwestern states didn’t force creation on the .450 round, legislation, as it often does, pushed innovation on that platform.

Read more here