Doubt on Action reaction pair of Gun on a wood If I shoot wood  with a gun,If the force of bullet of more,it will pierce through the wood but if not it may get stuck inside it.When these two scenarios were happening,We say an action reaction pair will happen between those two bodies which is equal and opposite to the action force then,Why didn’t the normal reaction stop that body at first instant if the action reaction pair were equal there?
 A: I am not sure whether I understood exactly what you are saying . That's why, I will answer by taking two cases and I think one of them is what you asking for.

CASE 1 : I think you are asking that why doesn't the bullet stop the very instant it comes in contact (since it experiences some force) with the wood from mathematical view.
Suppose you are standing on the middle of a road and a massive truck is approaching you . You have decided to stop the truck by pushing it back.
When your hand comes in contact with the truck , you apply a force on that truck in opposite direction of its velocity and in return the truck also applies a force on you in the opposite direction of your applied force. That's why, you start sliding on that road but did the truck stop the very instant you came in contact with the truck ?
No , and it can't be even possible since :
$F = m\frac{\delta v}{\delta t}$.
If $\delta t$ tends to zero , $F$ tends to infinity , which isn't possible.
Same is with the case of bullet and wood and the bullet can't stop at the instant it comes in contact with the wood.

Case 2 : I think you are confused with action- reaction pairs .
When bullet comes in contact with the wood, it applies a force on that wood (normal force) and from Newton's third law , we know that the wood also applies the same amount of force on that bullet . So there is just a single force acting on the bullet as well as on the wood .
Both the normal force are acting on two different objects and not on a single object and because of this you can't say that both the action and the reaction force should cancel each other and the bullet shouldn't pierce through the wood .

Yes one more thing, if you take the bullet and the wood as a system then the action- reaction  pairs cancel each other out (since by taking them as a system , you are not considering them as two different objects) but this doesn't mean that the bullet will not move or pierce , it only means that the center of mass of the ( bullet and the wood ) will not experience any change of its state.
FBD for bullet :

FBD for wood :

Hope it helps ☺️.
