The description of the original Thompson submachineguns has to include a description of the unique locking system used in the design. That locking system is referred to as the Blish lock. Under high pressure, two angled pieces of different metal would adhere to each other more strongly that two pieces of the same metal. Conversely, two angled pieces of different metals would not “stick” together as well under low pressure. This does not mean that the two differing metals would actually adhere to each other, but that their coefficient of friction would be higher under high pressure rather than low pressure. The Blish locking piece used in the Thompson design was made of bronze and bared against two parts made of steel, making for the dissimilar metals required for the Blish Principle to work. In practice, the Blish Principle did little more than increase the difficulty in machining out the Thompson designs that used it. The Blish lock itself was complicated due a great deal to the very specific angles that had to be machined into the part. During World War II, it was dropped completely in the redesign of the Thompson into the M1 and M1A1 models which operated from on the simple blowback principle.
