It was.
The live-fire prop was built on a C96 Mauser and appeared in the hands of Frank Sinatra in the 1966 film
The Naked Runner, then was slightly remodified for
Star Wars a decade later. There were two distinct versions used in the original film, one for the scenes filmed in England and Tunisia and another nonfiring version built on a dummy gun used only in pickup shots filmed back in the US (when Han draws his pistol in the cantina).
Then there was a different one built on a dummy Mauser used for
The Empire Strikes Back, which Luke had a similar one plus a stunt version mostly used for Han, and then a THIRD version likewise built off a dummy pistol used for
Return of the Jedi. For the sequels and the Han Solo movie, they rebuilt a version of the original prop using an airsoft M712
Schnellfeuer Mauser.
For some more pics & prop info:
ANH Hero:
http://www.partsofsw.com/dl44sw.htm ANH Pickup:
http://www.partsofsw.com/dl44sw_alt.htm ESB Han:
http://www.partsofsw.com/dl44hesb.htm ESB Luke:
http://www.partsofsw.com/dl44lesb.htm ESB Stunt:
http://www.partsofsw.com/RangedWeapons/dl44stunt.htm ROTJ Han:
http://www.partsofsw.com/dl44rotj.htm 
