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The piston must be able to supply enough force to break the shear pins with a 4x ?x safety factor before material deformation (this will allow for a smaller, but significant, safety factor on piston use in-flight). This safety factor was chosen using the 1974 NASA Aerospace Pressure Vessel Safety Standard [3].
As of 12/27/2017, we plan on using 180lbs of shear pins, making for a desired yield force of 720lbs. This analysis makes use of thin-walled pressure vessel theory [2], paraphrased below:
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[2] University of Colorado, Boulder: Thin-Walled Pressure Vessel Theory
[3] NASA Aerospace Pressure Vessel Safety Standard, 1974: NSS/HP-1740.1
Note that this standard was cancelled in July, 2002.