I know this is an old thread but it came up when I googled to see if there was an official answer to a similar situation. Here is what I suggested as a player and my DM agreed.
1.) The weapon delivering the spell follows normal crit rules for that weapon.
2.) Spell damage also threatens on a natural 20 and does x2 if successful just as if it had been delivered normally.
3.) Shocking Grasp, if the attack misses but the +3 would have made it a hit only the spell hits. Imagine the weapon striking the armor with no effect but the electricity damage being conducted by the armor or the swing being so close that electricity arcs off the weapon. Use those kind of descriptions in your game.
1.) The weapon delivering the spell follows normal crit rules for that weapon.
2.) Spell damage also threatens on a natural 20 and does x2 if successful just as if it had been delivered normally.
3.) Shocking Grasp, if the attack misses but the +3 would have made it a hit only the spell hits. Imagine the weapon striking the armor with no effect but the electricity damage being conducted by the armor or the swing being so close that electricity arcs off the weapon. Use those kind of descriptions in your game.
