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Expert Answer
.450 Bushmaster can also be used in standard milspec AR-15 upper receivers (albeit with an enlarged ejection port), so the .450 Bushmaster round works well with standard M4 feed ramps. To save money, most companies making a .450 Bushmaster upper receiver simply take an AR-15 upper receiver and mill out the ejection port rather than having to custom-make a completely different upper receiver. This saves them a great deal of money in production costs.