The Blish Lock is a breech locking mechanism designed by John Bell Blish based upon his observation that under extreme pressures, certain dissimilar metals will resist movement with a force greater than normal friction laws would predict. In modern engineering terminology, it is considered to be an extreme manifestation of what is now called static friction, or stiction. His locking mechanism was used first in the Thompson submachine gun.
The Blish Lock was the result of observations made by Blish of large naval guns. Blish observed that the breech blocks of naval guns with interrupted thread breechs remained closed when fired with full charges but tended to unscrew when fired with light charges. Using his mathematical and analytical training, he concluded that dissimilar metals have a tendency to adhere to each other when subjected to very high pressure. This principle of metallic adhesion of dissimilar metals became known as the Blish Principle. It did not take Blish very long to put this knowledge to use in a delayed-blowback breech lock. He developed a working model that used a simple wedge as the lock, and was eventually assigned U.S. Patent 1,131,319 on March 9, 1915.
While this system was used in the Thompson, it was eventually found to be redundant. In fact, the Thompson was later redesigned as a simple blowback weapon (the M1/M1A1), and worked perfectly well. Some authorities, such as Julian Hatcher, feel the Blish Lock as employed in the Thompson did not accomplish anything in terms of actual breech locking. Any real advantages to the system were far outweighed by the additional cost of manufacture associated with the device. Also, incorrect installation of the Blish lock can render a Thompson inoperable.