A sample cylinder for holding natural gas or other fluid samples for analysis which can completely clear itself of one sample before being filled with a subsequent sample. The piston is adapted to receive an elastomeric head which is preferably concave to the end cap. The end cap is also preferably sloped downward toward the outlet. When the piston head contacts the end cap, contact begins near the cylinder body wall and progresses inward to the outlet. The end cap is adapted to receive an elastomeric gasket seal instead of an o-ring. No spaces are left between the end cap and the cylinder body which could retain sample fluid.