Damage Reduction, Sorta and energy weapon missfires

In the home game on the Orbital, there are these very large maintenance robots (more on them later, all their secrets arnt out yet)

In order to simulate the heavy plating on them i have a quirky home-rule. A character rolls to hit them normally but when it comes to damage rolls, they must do at least 5 points of damage for the damage to go through. now these things have been shot, grenade blasted, stabbed, picked up with telekinesis and dropped and all kinds of different violence.  I also have an odd ruling on low rolls

example, Spaceman Biff shoots his trusty laser rifle on  burst fire, he gets three shots. he rolls a 17 an 13, and  a 3.  since he is firing on burst fire (my rule) any roll of a 1,2 or 3, (times shot) is a weapons foul. energy cell depleted, barrel fouled, however, but the first two shots went off and hit. this was done so that people thought about odds to miss better when spraying out laser bolts. I have a similar mechanic on the heavy machine gun user but he gets to roll a d10 for chances of belt jam or missfire, being whatever he rolls number wise is the number of times that he can missfire. we use a cone effect for the HMG, evade save or damage, hits everything

now  Spaceman Biff rolls damage, he rolls a 4, and a 9, to simulate the sheer hard to kill of these things anything under a 5 is glancing , the 9 goes all they way through.  In a game where shooting robots is the main for of entertainment for a few of the players, this has worked so far in making the harder to wound but still killable as they are a main target.  So its not really DR, its potential DR

is this a fiddly mechanic, i originally did it as easy way for me to track damage as i didn't want to worry about small numbers when the weapons where capable of doing such large damage and multiple times.  

Thoughts?