A police source said on Sunday the identities of the victims were unclear and the bodies appeared to have been there for some time.
They were found on Friday and are now in a Baghdad mortuary, another police officer said.
Such finds have become a grim routine of the violence in Iraq. In the couple of months since a Shia-led government was formed, more than 850 people have been killed, fuelling fears that the nation could plunge into civil war.
Anti-US fighters have stepped up attacks on US and Iraqi security forces and civilians, while Sunni leaders have accused Shia militias and government forces of attacking ordinary Sunnis - a charge the various groups and ministries deny.
A leading Sunni organisation, the Association of Muslim Scholars, issued a statement on Sunday saying that 30 bodies had been found at the firing range.
It said one body was identified as belonging to a Sunni Arab, but it gave no details.