Actions: Blowback Action: Blish Lock

In our last few of posts, we've studied the basic principles of a blowback action and the reason why we want to delay the blowback action. In our last couple of posts, we saw that mechanical means can be used to delay the movement of the bolt. In this post, we will look at another way to delay the movement of the bolt -- a method that uses static friction, the so-called Blish lock. The Blish lock is used in one of the most famous weapons invented in the early 20th century, the weapon that gave rise to the term "submachine gun", the one and only Thompson submachine gun, a.k.a the "tommy gun".

The Blish lock was invented by John Bell Blish, a career officer in the US Navy. While he was serving aboard a US Navy warship, he noticed a very interesting fact about naval guns. The guns in question were breech loading with screw on type breech blocks. He noticed that if a gun was fired with a normal round with a full load of gunpowder, the breech block would hold in place perfectly, but if it was fired with a training round which has a lot less gunpowder in it, the breech block would tend to unscrew itself and the case would fall out. After a lot of analysis about this phenomenon, he came to the conclusion that certain dissimilar metals tend to stick to each other if very high pressure is applied. This principle of metal-to-metal adhesion is now known as the "Blish principle." This adhesive force is really due to static friction between the two metallic surfaces. While he was not entirely certain about the physics behind the phenomenon, he did nevertheless note down that it happened and applied it to designing a wedge shaped blowback action that he patented in 1915. The action consisted of two diagonally sliding dissimilar metal wedges on the back of the bolt. Due to the pressure of the gases and the Blish principle, these two metals would adhere to each other initially. This extra resistance slows down the backward movement of the bolt. It was noticed much later that the Blish lock was not as effective as proclaimed, and the same effect could be obtained by merely increasing the weight of the bolt by one ounce and making it work on the principle of straight blowback.

While the Blish lock patent itself went nowhere initially, the next development came due to a retired US Army General, John T. Thompson, and the onset of World War I. The standard US military rifle at that time was the venerable bolt action M1903 Springfield rifle and the standard pistol was the Colt M1911. Interestingly, Gen. Thompson had earlier served as the chief of the Small Arms Division of the Ordinance department and had supervised the development of the Springfield M1903 rifle and approved the selection of the Colt M1911 pistol.

During World War I, trench warfare became the standard method of fighting and it became clear that there was a need for a weapon with a high rate of fire that could be used to clear a trench of enemies. General Thompson was looking to replace the bolt action rifle with an auto-loading one, a concept he called the "trench broom." He was aware of gas operated and recoil operated actions in his day, as these were in use for medium and heavy machine guns, but these used many heavy moving parts and were not as reliable then. He researched the straight blowback actions of the day, which were only suitable for really low powered ammunition. While he was studying various blowback actions, he came across the Blish patent and decided to use it instead. Accordingly, he arranged a meeting with John Blish and offered him some stock in the new weapons company he was planning to form, in exchange for the manufacturing rights of the Blish patent.

With Blish as a partner, Thompson secured some venture capital and formed the Auto-Ordnance Corporation in 1916. He also managed to hire Theodore Eickoff, who was once Thompson's assistant when he was running the Army Ordnance department. He also found an unemployed railroad fireman with mechanical aptitude called George E. Goll and hired him to be Eickoff's assistant. These two men were the principal designers of the Thompson submachine gun. Later on, Oscar Payne also joined and added some key Tommy gun features, such as the self-oiling mechanism and its distinctive drum magazine.

The original plan was to develop an auto-loading rifle using the military .30-06 round, which is the same round that the Springfield M1903 rifle uses. However, a series of problems were discovered with the Blish lock. For one, it could not handle such a powerful round and would wear out prematurely. Worse, the extraction of the fired cartridge case would not work unless the cartridges were lubricated. Eickoff did some research and found that the only military cartridge that would work reliably with the Blish lock was the .45 ACP cartridge, which is the cartridge used for the Colt M1911 pistol. Eickoff dreaded telling Thompson the bad news, but to his surprise, Thompson took the news very well and said "Very well, we will put aside the rifle for now and instead build a little machine gun. A one-man, hand held machine gun. A trench broom!" Thompson had realized what many European generals of the era hadn't, that nineteenth century warfare tactics didn't mix very well with twentieth century weapons. The traditional cavalry offensive charge was no match for heavy machine guns in trenches and the war had stalled with heavy casualties and little progress. While machine guns of the day were great for defense, they were too large and too heavy to be used for offensive actions. What Thompson visualized was a hand-held weapon with enough firepower, that could be used for hit-and-run tactics to clear enemy trenches. Accordingly, he directed Eickoff to develop a class of firearms that had never existed before then.

By the summer of 1918, all problems had been solved and the new class of weapon was called the "Annhilator Mark I". The first batch of weapons destined for Europe was delivered to the New York harbor on November 11th 1918, the very day that the armstice was signed in Europe, signalling the end of World War I. Suddenly, Thompson was stuck with a weapon that didn't have any demand! However, he was not a man to be let down by this setback and directed Auto Ordnance to redesign it for civilian use, which they did by 1919. Seeking a new name that could be used to distinguish it from its larger and heavier machine gun ancestors, he came up with names like "Autogun" and "Machine pistol", before coining the term "submachine gun". The rest, as they say, is history.

Two views of the Blish lock (note the brass content). Note the wedges along the center and the sides of the H shaped lock.

Bolt, Blish lock and actuator of a Tommy Gun. Note the dissimilar metals used

Disassembled Blish Lock, Bolt and Actuator

Tommy Gun with straight box magazine

Tommy Gun with rotary drum magazine

The Tommy gun was initially marketed to the police. The improved model M1921 was also offered for sale to the general public. It was a very finely machined weapon with high quality components and consequently commanded a high sale price of $200 (for contrast, a Ford car of that era cost only $400). It really came into the public eye during the Prohibition era, when both gangsters and law enforcement began to heavily use it. It acquired such nicknames as the "Tommy Gun", "Chicago Piano", "Typewriter", "Chicago Typewriter", "Chopper", "Broom" etc. Due to its notoriety, it was the main reason for the passage of the National Firearms Act of 1934 in the US. It was also used by the US marines in several smaller conflict and was adopted by the US military in 1938. It was used in World War II, Korea, Vietnam and beyond. It still turns up in some present day conflicts occasionally, such as the Bosnian war of the 1990s.