In various embodiments, a surgical stapling head is provided that may comprise a staple cartridge for supporting one or more staples, a core movable relative to the staple cartridge, at least one staple driver extending from the core, and a casing configured to at least partially hold the staple cartridge and movably receive the core and the staple driver(s). The casing may further comprise at least one retention member that is configured to move from a first position to a second position when sufficient external force is applied to the retention member, such as that provided by a shaft of a surgical stapler during insertion of the stapling head assembly into the shaft. When the retention member(s) are at the second position, the staple driver(s) may be prevented from driving staples from the staple cartridge, thereby providing a firing lockout feature to the stapling head assembly during insertion into at least a portion of a surgical stapler.