You can use any of these textures to replace the skins on the ACR.

To replace a skin, name your chosen texture as "acrDesert.dds" for the Desert Skin, "acrUrban.dds" for the Urban skin, "acrWoodland.dds" for the woodland skin, or "acr.dds" to replace the default skin and put it in the "textures/weapons/ACR" folder in your Fallout 3 Data directory.