Working in a law enforcement field, this is the first logical comment I have seen. Now not only does everything you said have to be applied, but also one has to remember that when justifying any particular use of force all the LEO has to prove is that the force used would be comparable to that used by any other person out there. With that said think of this situation this way: Maybe that object that possibly resembled a pipe bomb was put there by someone else other than the OP, and that it was really going to explode. With that in mind, think about this: On a motor vehicle what is the part that will provide the most cover from projectiles? Usually projectiles equates to bullets but in this case the projectiles would be whatever was packed into that pipe bomb. Maybe ball bearings, shards of glass, tacks, or nails. The answer to this question is either the engine or the wheel. In this case, seeing the supposed pipe bomb came from the glove compartment, which happens to be on the opposite side of the engine compartment/engine, what the LEO is said to have done actually makes a lot of sense. Someone who suddenly finds himself/herself staring at an object that they think will explode very well may act like this in the same exact circumstances. Now given the small chance that it really was a pipe bomb put there by someone else that really did explode, I'm sure OP wouldn't have any issues about being slammed up against the hood of the car. Although I would be willing to bet OP is withholding more than what is being told, if this is even an event that truly happened, I would still consult another lawyer.

I would assume they caught a glimpse of it as OP was reaching for the registration/proof of insurance

oh see that makes sense.

I agree with you. Why would they be able to see what is inside OP's glove compartment? OP must have done something to get their car searched, unless it was spotted when he opened it to get his registration.