For an AR-15 chambered in .450 Bushmaster with a collapsible stock, we recommend the KynSHOT AR15 carbine hydraulic buffer (part# RB5000) and the SPRINCO Extra Power AR-15 carbine buffer spring (color code RED – Part# SPRINCO-25004. This combination has been used by many of our customers with the .450 Bushmaster with excellent results.
https://www.blitzkriegcomponents.com/hydraulic-kynshot-recoil-buffer-223-carbine.html