Midway has a sale on complete uppers for the BM450 for about $280. The handguard is a bit funky, so I would do an upgrade, but it is good to go as is. 16" barrel.
https://www.midwayusa.com/product/1019500940?pid=466691&utm_source=super-sonic-sale&utm_medium=email&utm_content=main-image-link&utm_campaign=super-sonic-arstoner