Period-24 glider gun is a true period 24 gun discovered by Noam D. Elkies in 1997. It uses four large  period 4 oscillators (two of them variants of fountain, the other two adjoined by common casing) to hassle two T-tetrominos, which react together to produce gliders. The form on the right is a reduced variant found by Karel Suhajda. The eater 1 can be replaced with a  unix for compacting purposes, such as in the period-48 glider gun.
No more than five days after the initial discovery, Elkies found a method to double the period of the gun, resulting in the first known period 48 glider gun.
A smaller version was found in 2016 by Tanner Jacobi, using  Rich's p16 to double the period.
Original period 24 glider gun.
Period 48 glider gun.
Period-20 glider gun
↑ 1.0 1.1 Paul Callahan (February 11, 2001). "
p24 "true" gun (mail digest)". Retrieved on March 17, 2016.
↑ Jason Summers (December 30, 2012). "
Pattern Collections, jslife/osc". Retrieved on March 17, 2016.
↑ Tanner Jacobi (2016-07-05). "
Re: Soup search results". ConwayLife.com forums. Retrieved on 2016-07-05.