The volume \( V \) of a rectangular prism is given by the formula:
\[ V = \text{length} \times \text{width} \times \text{height} \]
For the rectangular prism in question, we know that its volume is \( 252 , \text{in}^3 \).
The volume \( V \) of a rectangular pyramid is given by the formula:
\[ V = \frac{1}{3} \times \text{base area} \times \text{height} \]
For the rectangular pyramid that has a base and height congruent to the prism, its base area is equal to the area of the base of the prism, and the height is the same as the height of the prism.
Since the base area of the prism is \( \text{length} \times \text{width} \), we can express the volume of the pyramid as:
\[ V_{\text{pyramid}} = \frac{1}{3} \times (\text{length} \times \text{width}) \times \text{height} \]
Given that the volume of the prism is:
\[ V_{\text{prism}} = \text{length} \times \text{width} \times \text{height} = 252 , \text{in}^3 \]
We can rewrite the pyramid's volume as:
\[ V_{\text{pyramid}} = \frac{1}{3} \times V_{\text{prism}} = \frac{1}{3} \times 252 , \text{in}^3 \]
Calculating this, we get:
\[ V_{\text{pyramid}} = \frac{252}{3} , \text{in}^3 = 84 , \text{in}^3 \]
Thus, the volume of the pyramid is
\[ \boxed{84 , \text{in}^3} \]