Ergonomic Design. The vertical struts for the MBA-3 are fixed therefore the cheek piece is not reversible like the MBA-1 however the cheek piece itself is designed with a cheek pad for left-handed use. The MBA-3 and MBA-4 Carbine buttstocks are not designed to be used with Large Bore Calibers such as .338 Lapua or .50 BMG applications. Kit includes carbine buffer tube carbine buffer spring latch plate and lock ring