A containment cap assembly for enclosing a metallic fastener extending through a structure, comprising: an inner cap configured to enclose a portion of the metallic fastener, said inner cap including a projection extending from an outer surface of the inner cap; and an outer cap comprising: a sidewall having a first end which defines an opening and an inner surface which defines a first space, wherein: the opening is in communication with the first space; a dimension of the opening and a dimension of the first space defined by the inner surface of the outer cap are each greater than a dimension defined by the outer surface of the inner cap such that with positioning the inner cap within the outer cap, a second space is defined between the inner surface of the sidewall of the outer cap and the outer surface of the inner cap; and a slot defined by the inner surface of the outer cap which extends along the inner surface away from the first end of the outer cap in a first curvilinear direction relative to the first end, wherein, when the inner cap is inserted into the outer cap, the projection is configured to be positioned within the slot and to move in the first curvilinear direction within the slot while the outer cap turns relative to the inner cap. an inner cap configured to enclose a portion of the metallic fastener, said inner cap including a projection extending from an outer surface of the inner cap; and an outer cap comprising: a sidewall having a first end which defines an opening and an inner surface which defines a first space, wherein: the opening is in communication with the first space; a dimension of the opening and a dimension of the first space defined by the inner surface of the outer cap are each greater than a dimension defined by the outer surface of the inner cap such that with positioning the inner cap within the outer cap, a second space is defined between the inner surface of the sidewall of the outer cap and the outer surface of the inner cap; and a slot defined by the inner surface of the outer cap which extends along the inner surface away from the first end of the outer cap in a first curvilinear direction relative to the first end, wherein, when the inner cap is inserted into the outer cap, the projection is configured to be positioned within the slot and to move in the first curvilinear direction within the slot while the outer cap turns relative to the inner cap. a sidewall having a first end which defines an opening and an inner surface which defines a first space, wherein: the opening is in communication with the first space; a dimension of the opening and a dimension of the first space defined by the inner surface of the outer cap are each greater than a dimension defined by the outer surface of the inner cap such that with positioning the inner cap within the outer cap, a second space is defined between the inner surface of the sidewall of the outer cap and the outer surface of the inner cap; and the opening is in communication with the first space; a dimension of the opening and a dimension of the first space defined by the inner surface of the outer cap are each greater than a dimension defined by the outer surface of the inner cap such that with positioning the inner cap within the outer cap, a second space is defined between the inner surface of the sidewall of the outer cap and the outer surface of the inner cap; and a slot defined by the inner surface of the outer cap which extends along the inner surface away from the first end of the outer cap in a first curvilinear direction relative to the first end, wherein, when the inner cap is inserted into the outer cap, the projection is configured to be positioned within the slot and to move in the first curvilinear direction within the slot while the outer cap turns relative to the inner cap.