Rambo fires the 1911 at the River pirates.
The Colt M1911A1 is a type of .45 ACP semi-automatic pistol that can hold eight rounds. It is one of Rambo's only weapons he kept from his Vietnam days, other than his knife and standard-issue Zippo Lighter.
History
It was designed by American firearms inventor John Browning and is the oldest service pistol in the world, still used in over 25 nations for over 100 years; it has proven itself to be one of the most popular and timeless handguns ever made. The 1911 has inspired countless copies and similar designs, including the Beretta 92, the SIG-Sauer P226, the CZ-75 and the Walther PP.
First Blood
There was an alternate ending to First Blood that featured John Rambo begging Colonel Sam Trautman to kill him after ranting about the horrors of war. Tratuman refused, and Rambo grabbed Trautman's Colt M1911A1 pistol and shot himself in the chest with it. Test audiences thought this ending to be too much of a downer, so they changed it to the ending where Rambo surrenders to the police outside the station instead.
Rambo: First Blood Part II
A Vietnamese river pirate is briefly seen with a Colt M1911 in a hip holster.
Novelization
It is mentioned that Colonel Trautman carries a ".45" in a holster on his hip. Although it wasn't specified, presumably this is a Colt 1911A1, which was the standard issue sidearm for all military personell in the 1960s and 1970s, when Trautman was in the army. It was also mentioned that river pirates were armed with them and that Rambo himself carried another .45, likely a 1911 as well. The .45 was not used in many pistols of the time as well.
Rambo IV
While Rambo is taking the missionaries down the Salween River on his boat, they are stopped by a band of Tatmadaw river pirates, who demand Sarah in exchange for passage. Rambo knows that they will rape her, and he attempts to offer them money, but they do not accept. When other attempts at negotians fail, Rambo pulls his Vietnam service G.I. .45 1911 and kills all of the river pirates, later stepping over and promptly executing a wounded soldier that is reaching for his Tokarev TT-33 pistol.