A piston retainer plate includes a first end, a second end opposite the first end, a piston pocket extending from the first end to the second end, a straight annular surface extending from the first end, an angled annular surface extending at an angle from the second end, a curved annular surface connected to the straight annular surface and the angled annular surface, the curved annular surface being between the straight annular surface and the angled annular surface, and a curved vent within the curved annular surface. The straight annular surface, the angled annular surface, and the curved annular surface define the piston pocket.

A piston retainer plate includes a first end, a second end opposite the first end, a piston pocket extending from the first end to the second end, a straight annular surface extending from the first end, an angled annular surface extending at an angle from the second end, a curved annular surface connected to the straight annular surface and the angled annular surface, the curved annular surface being between the straight annular surface and the angled annular surface, and a curved vent within the curved annular surface. The straight annular surface, the angled annular surface, and the curved annular surface define the piston pocket.