Hillingdon Borough Police tweeted a picture of what looks like a revolver on a car seat on Friday evening (October 13).
The tweet said: "This is what police officers are dealing with on a daily basis in Hayes.
"Another one off the street in an attempt to keep people safe."
The gun in the photograph does not appear to be a genuine firearm, but rather a plastic replica.
However, as police from Barnhill also tweeted, "the trouble is an officer won't know until it's been safely disarmed and inspected.
"Looks real enough out on the streets."
Crime figures for 2016/17 showed increases in knife and gun crime in all west London boroughs, despite an amnesty in February 2017.
A total of 83 firearms were handed in during the amnesty, including an AK47 and a shotgun .
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