Pressure tsi

please, what does the unit of pressure “tsi” mean? For example, for 380 rev. is 10 tsi, how much is MPa?
Thank you

most logical would be “Ton per Square Inch”

Tons per square inch is the way the British express gas pressures. Due to their very different method with an oiled case acting backwards on a crusher in the bolt head, these pressure figures cannot be simply converted to the results of other methods.
Having said that, based on a figure in the 1929 Textbook of Small Arms, numerically 1 tsi equals 15.445 MPa, if I got the math right.

thank you gentlemen