https://www.americanhunter.org/content/behind-the-bullet-400-legend/

Behind the Bullet: 400 Legend

https://www.americanhunter.org/media/k4cibfii/btb-400-legend-lead.jpg

by PHILIP MASSARO posted on February 26, 2024
NEWS, GUNS & GEAR, AMMO

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BTB 400 Legend Lead
Give an American a new set of laws, and before the ice melts in your drink, someone will have an idea to work around it. With the numerous and varied laws regarding what makes a legal deer cartridge in many of the Midwestern states, a good number of cartridges have been relied upon or developed to meet as many of the criteria as possible. Among the popular choices were the 450 Bushmaster, the .45-70 Government, the .444 Marlin and the .38-55 Winchester; more recently the 350 Legend and 360 Buckhammer were specifically developed to adhere to as many of those parameters as possible.

Released at the 2023 SHOT Show, Winchester/Browning’s 400 Legend took their previous 350 Legend concept further, giving the shooting public a straight-walled cartridge, fully suitable for both a bolt-action rifle as well as the AR platform, conforming to the collective statutes set forth by a number of those Midwestern states.

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I'm not really sure of what purpose this cartridge serves, other than a cartridge fitting into a AR.