Once you start TranZit do not buy any guns off the wall, you must have no weapons.

Get to the diner an do not hit the box, but open up the doors and build the shield... Do not take a shield - it's probably easier not to.

Get the perks you want and open up pack-a-punch and build it, then put the hand gun that you have from the start into pack-a-punch and do not take your gun... You should have no weapons.

Get back to the diner and grab the riot shield. It will be unbreakable due to the fact that they cannot leave you without a weapon - they compensate with the riot shield!