Crimestoppers is urging Oxfordshire residents to report friends and relatives who ignored last month's gun amnesty.
During the national campaign, 1,185 guns were surrendered in the Thames Valley Police area, as well as 23,877 rounds of ammunition.
Those handing in weapons could do so without fear of prosecution.
But the amnesty is now over and Thames Valley Police has warned there will be no second chances for anyone caught with illegal guns
The force has pledged to come down hard on offenders.
Crime-fighting charity Crimestoppers has appealed to anyone with information about illegal weapons to name people who did not take up the amnesty offer.
Roy Trustram Eve, the chairman of Thames Valley Crimestoppers, said: "We need information about friends, neighbours, family members or anyone else who has an illegal gun.
"No caller is asked for their name. If the information they give leads to someone being arrested and charged with possession of an illegal firearm, the caller may receive a reward."
In the north Oxfordshire police area, 155 guns were handed in during April, including 53 prohibited weapons, the second highest in the Thames Valley force. In south Oxfordshire, 144 guns were handed in, including 28 prohibited weapons. Fifty-eight guns were surrendered in Oxford, 14 of which were banned.
Prohibited firearms are guns, such as handguns, which cannot be legally held. Other guns, such as shotguns, can be held with a licence.
Crimestoppers' phone number is 0800 555111.
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