Officers raided the property as part of an investigation into children being exploited by organised crime groups.
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The man, aged 47,arrested at the scene on suspicion of modern slavery offences and possession of a firearm. He has been taken to custody where he remains for questioning.
He is the fourth person to be arrested by officers since the child exploitation investigation began five weeks ago.
Police say boys as young as 14 could be being exploited as part of organised crime.
So far, three men - aged 38, 33, and 28 - have also been detained on suspicion of modern slavery offences and have since been released on bail pending further enquiries.
Officers from Stockport's Challenger team have spoken to two teenage boys as part of the investigation - both of whom have since been safeguarded with support from Stockport council.
Detective Sergeant Lee Attenborough, of GMP Stockport's Challenger team, said: "Today's find may be a concerning one for members of the local community, but it is an absolutely vital one in ensuring that this potentially lethal weapon is out of harm's way and with our specialist officers for forensic examination.
"We're conducting a fast-paced and detailed investigation into the suspected criminal exploitation of children, where we have already arrested four men and are ensuring we are working with our local partners to offer all possible support to the two boys we have safeguarded as part of this investigation.
"At this stage, we believe there aren’t any further individuals involved in this group; however, we are keen to ensure that if there are any further victims of criminal exploitation in Stockport, that they contact police in confidence knowing that we will pursue all available lines of enquiry and ensure their safety as this is our top priority."
Enquiries are ongoing and anyone with concerns or suspicions about the criminal exploitation of young and vulnerable people are asked to contact police online or via 101, or the independent charity Crimestoppers anonymously on 0800 555 111.
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