Blish-Lock Thompson1928a1

Thompson M1928A1 bolt group with "H" type Blish-Lock piece

Thompson 21 and Rifle

Thompson Autorifle Model 1923 (top, upright) chambered in 7.62x54mmR and SMG Model of 1921 (bottom, inverted) are both examples of firearms that used the Blish lock.

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 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 resulted from Blish's observation of large naval guns. He noticed 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. Blish 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 US patent 1131319 on March 9, 1915.

The most famous applications of the Blish Lock were the Thompson Autorifle and Thompson submachine gun. Several engineers suspected the autorifle functioned more as a retarded blowback than as an adhesion-locked breech action.[1] Some authorities, such as Julian Hatcher, felt the Blish Lock as employed in the submachine gun did not accomplish much in terms of actual breech locking. In fact, the submachine gun was successfully redesigned as a simple blowback weapon (the M1/M1A1). Any real advantages to the system were far outweighed by the additional cost of manufacture associated with the device. Also, in the Thompson submachinegun the "H" shaped bronze lock connects the bolt actuator to the bolt body; incorrect installation of the Blish lock can render a Thompson inoperable.


  1. Julian S. Hatcher, Hatcher's Notebook, Military Service Publishing Co., 1947, pages 44-46.

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