Abstract: A surgical instrument includes a stapling head assembly and a shaft assembly having a distal end portion configured to removably couple with the stapling head assembly. The stapling head assembly has a cartridge housing, a staple deck, a plurality of staples that includes first and second staples, a staple driver with a first driver portion and a second driver portion configured to respectively receive the first and second staples thereagainst, a cylindraceous knife, a trocar, and an anvil. The first and second driver portions are configured to respectively direct the first and second staples against the anvil to form the first staple with a first formation height and the second staple with a different, second formation height. The shaft assembly has a safety mechanism configured to prevent firing of the staple driver and the cylindraceous knife when the force load received is below a predetermined minimum force.