Blish lock

The Blish Lock is a friction-lock designed by John Blish, that takes advantage of the fact that under extreme pressures, materials will resist movement through friction greater than normal friction laws would predict (which are proportional to pressure - force and area). His locking mechanism was used first in the famous Thompson Submachine gun, or Tommy Gun; tests prior to the development of the Thompson showed that the locks experienced great wear when firing full-power rifle cartridges, and that reliability was marginal (although this problem could likely have been fixed by fluting the chamber, as is commonly done in a delayed-blowback firearm).

The Blish Lock was the result of observations made by John Bell Blish of large Naval guns. He noticed that guns firing relatively light charges tended to have their breech blocks unscrew and fly open. But the breech blocks of guns firing heavy charges remained closed. Using his mathematical and analytical training, he concluded that metals have a tendency to adhere to each other with a force much greater than friction, when subjected to very high pressure. This principle of metallic adhesion has since come to be known as the Blish Principle. It didn't 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 awarded a patent.

(Thanks to http://www.nfatoys.com/tsmg/ )