It wasn't armor plate, it was just a cover panel. The protection would be built into the part that the panel covered (i.e. the clear panel was bullet resistant).
The 103 and 108 may have had a similar projectile size, but they were nothing alike. Not in ballistics, not in trajectory, not in accuracy. The barrel on the 103 was very short, and on any gun shorter barrels mean less accuracy. Just look at snub-nose revolvers as compared to 4", or 6" revolvers. The slower travel time also means the round has more time to drift. Since the 103 gets to the target that much faster, its got less chance of being a mile off like the 108 in-game is.