Looking for a 1960s book about people who think they are first training for and then fortifying on a planet where all the vegetation and other life is deadly and actively trying to kill intruders. Plants actively try to eat through the walls of their fortress, for example. The weapons people use are connected to their reflexes somehow.
Harry Harrison’s Deathworld trilogy.
It features deadly plants and "forearm mounted automatic holsters".
The planet was called Pyrrus...a strange place where all the beasts, plants and natural elements were designed for one specific purpose: to destroy man.
The settlers there were supermen...twice as strong as ordinary men and with milli-second reflexes. They had to be. For their business was murder...
It was up to Jason dinAlt, interplanetary gambler, to discover why Pyrrus had become so hostile during man's brief habitation...