A surgical stapling instrument, comprising: a reload including a staple cartridge assembly and an anvil assembly (306; 3012) moveable with respect to one another; an adapter assembly having an elongate shaft and an articulating distal end, a mounting member being disposed at the distal end and being pivotably attached to the elongate shaft, the mounting member having a connection feature for removably and replaceably attaching the reload to the adapter assembly; a drive mechanism in the adapter, the drive mechanism including a bar extending through the mounting member and having a beam at a distal end of the bar, the bar being flexible and supported by a pair of bar guides located at the distal end of the elongated shaft, one of the bar guides alongside a first side of the bar, and the other of the bar guides being alongside a second side of the bar, the bar being further supported by a pair of blowout plates, one of the blowout plates being alongside the first side of the bar, and the other of the blowout plates being alongside the second side of the bar, the pair of blowout plates being adjacent the pair of bar guides and extending distally of the pair of bar guides to the mounting member, the bar being further supported by a support block having an opening, the bar and the pair of blowout plates being disposed in the opening of the support block, the bar, the pair of bar guides, the pair of blowout plates, and the support block being pivotable from central positions in the adapter assembly to off-center positions in the adapter assembly, wherein when the instrument is articulated, the distal ends of the bar guides pivot off-center in the opposite direction to the mounting member. a reload including a staple cartridge assembly and an anvil assembly (306; 3012) moveable with respect to one another; an adapter assembly having an elongate shaft and an articulating distal end, a mounting member being disposed at the distal end and being pivotably attached to the elongate shaft, the mounting member having a connection feature for removably and replaceably attaching the reload to the adapter assembly; a drive mechanism in the adapter, the drive mechanism including a bar extending through the mounting member and having a beam at a distal end of the bar, the bar being flexible and supported by a pair of bar guides located at the distal end of the elongated shaft, one of the bar guides alongside a first side of the bar, and the other of the bar guides being alongside a second side of the bar, the bar being further supported by a pair of blowout plates, one of the blowout plates being alongside the first side of the bar, and the other of the blowout plates being alongside the second side of the bar, the pair of blowout plates being adjacent the pair of bar guides and extending distally of the pair of bar guides to the mounting member, the bar being further supported by a support block having an opening, the bar and the pair of blowout plates being disposed in the opening of the support block, the bar, the pair of bar guides, the pair of blowout plates, and the support block being pivotable from central positions in the adapter assembly to off-center positions in the adapter assembly, wherein when the instrument is articulated, the distal ends of the bar guides pivot off-center in the opposite direction to the mounting member.