Mannequins were used for firearms practice at the fire-damaged Church of the Immaculate Conception in Qaraqosh, the biggest Christian town in the Nineveh Plains.
Manal Matti, who used to run a beauty salon nearby, told Catholic charity Aid to the Church in Need (ACN): "The jihadists used the church as a shooting range and the mannequins as targets.
"The mannequins are completely riddled [with bullets]."
"I do not know when I will ever be able to see the inhabitants of Qaraqosh coming again to my beauty salon."
Meanwhile, ACN claims St George's Syriac Catholic Church in Qaraqosh was used by IS as "an improvised bomb factory", and hundreds of bombs and grenades were found there.
Battle plans were scrawled onto church walls and chemicals were discovered, together with instructions about how to mix them into explosives.
Returning to Qaraqosh after being forced to leave in 2014, Father Sharbil Eeso told ACN: "Despite all the damage, I have hope for the future. If our security is guaranteed, Christians can continue to live in Iraq."
The 72 year old Catholic priest is one of the estimated 120,000 Christians forced from their homes following the IS invasion of Mosul and the Nineveh Plains in August 2014.
Louis Petrus, another Christian who has returned to Qaraqosh, said: "My house is damaged, most of my furniture has been stolen and my household effects are broken.
"The IS terrorists have destroyed a lot of my possessions, but I am still quite well off, considering the damage that I can see in my neighbours' houses.
"Many houses have been burned or even completely destroyed. I have been blessed."