A gun is a stationary pattern that repeatedly emits spaceships (or rakes) forever. By far the most common type of guns are glider guns, which emit gliders (the most well-known of which is the Gosper glider gun); however, guns that emit spaceships of other speeds, including c/2 orthogonal, 2c/5 orthogonal, and c/12 diagonal, have also been created.
The first gun to be discovered, the Gosper glider gun, was found by Bill Gosper in 1970, being the first known pattern in Conway's Game of Life to exhibit infinite growth. Since then, many guns have been constructed with various periods and that fire spaceships of various speeds. Due to simple glider syntheses of the standard spaceships many c/2 spaceship guns are known. The first gun that did not fire gliders or c/2 spaceships was a Cordergun (a gun that fires Corderships) constructed by Jason Summers in July, 1999 based on a synthesis of a variation of the 7-engine Cordership by Stephen Silver. In March, 2003 Noam Elkies completed the synthesis of 60P5H2V0 which Dave Greene used to construct the first 2c/5 spaceship gun in April of the same year.
In July, 2010 Dave Greene constructed a few guns for a geminoid spaceship. These are the first guns that shoot obliquely-traveling spaceships and at the time were the largest patterns constructed in Life, in terms of their bounding boxes.
On February 17, 2013 Josh Ball discovered a simple c/7 orthogonal spaceship, loafer. Adam P. Goucher found a synthesis for loafer later that day, allowing the construction of c/7 orthogonal spaceship guns.
It is not uncommon for glider guns to have multiple barrels; that is, streams of gliders that come out along multiple different paths. For example, the B-52 bomber is said to be double-barreled because it produces two streams of gliders (one toward the northwest and another toward the southeast). Gunstar and its variants all have four barrels, and P94S contains a whopping 12 barrels. Given a gun with multiple barrels, one can eliminate barrels by simply placing an eater 1 in the path of the unwanted glider streams.
A pseudo-period gun (as opposed to a true-period gun) is a gun that emits a period n stream of spaceships (or rakes) via a mechanism that oscillates with a period different from n -- this period will necessarily be a multiple of n. Pseudo period n glider guns are known to exist for all periods greater than or equal to 14, with smaller periods being impossible. The first pseudo period 14 gun was built by Dietrich Leithner in 1995.
A semi-true period gun is a gun whose actual period is double the output stream period, and similarily a tremi-true period, quadri-true and so on.
A true-period gun (as opposed to a pseudo-period gun) is a gun that emits a period n stream of spaceships (or rakes) via a mechanism that oscillates with period equal to n. True period n guns are known to exist for all periods greater than 53, but only a few smaller periods have been achieved, namely 20, 22, 24, 30, 33, 36, 40, 44, 45, 46, 48, 50, 52. Credits for gun periods below 62 are as follows:
|Period||Discoverer||Year of discovery|
|20||Matthias Merzenich and Noam Elkies||2013|
|22||David Eppstein and Jason Summers||2000|
|33||Arie Paap and Matthias Merzenich||2018|
|40||Adam P. Goucher, Jason Summers, and Matthias Merzenich||2013|
|50||Dean Hickerson and Noam Elkies||1996|
|52||Dave Greene, Chris Cain, Matthias Merzenich and Adam P. Goucher||2018|
|58||thunk and Matthias Merzenich||2016|
|59||Adam P. Goucher and Jason Summers||2009|
Guns in Life-like cellular automata
Due to the existence of some small spaceships in Life-like cellular automata with birth at two live neighbours (B2), many low-period spaceship guns have been found using tools such as WinLifeSearch. In contrast, there are very few outer-totalistic rules without B2 (and B0) that are known to contain guns. The following is a list of outer-totalistic rules with neither B0 nor B2 that have known guns:
- "Game of Life Status page". Retrieved on October 15, 2010.
- Adam P. Goucher (July 30, 2010). "Gemini guns". Retrieved on October 15, 2010.
- Dave Buckingham (October 12, 1996)). "My Experience with B-heptominos in Oscillators". Retrieved on May 14, 2009.
- Matthias Merzenich (Sokwe) (April 11, 2013). "Guns in Life-like cellular automata".
- Gun at Wikipedia