Shear pins are used to prevent the sections of a rocket from separating prematurely.

They are able to prevent drag separation or deploying a main parachute when the apogee charge goes off in a dual deploy rocket.

These are 1" long and can easily be cut to the appropriate length using a sharp knife on a hard surface.

4-40 shear pins are generally used in larger rocket with more mass that needs to be contained.

Always ground test to make sure that your deployment charge is strong enough to shear the pins.