The numbers here assume that the shooter merely looks down the sights and doesn't actually pause to aim (see Sighted Shooting, Tactical Shooting p. 13).
Aimed shots take an extra second to line up and add the weapons Acc to the roll to hit, you are also making an All-Out Attack (Determined).
Let's assume that Mr Glittershine has an average DX of 10 and his mom had taught him enough to get a point in his Guns/TL8 (Rifle). That gives him a skill level of 10. The rifle he used has an Acc of 5. The ranges were all less than 7 yards (-3).
So, what does that do to our round count?
A real live 20" barrel M16 has a RoF of 13, so only these first 13 benefit from the aiming. The number of shots gives a +3. So, 10+5+3-3. 15 or less to hit. Rcl is 2 and an average roll is a 10 on three dice so... 3 rounds tend to hit, ten are wasted. If he holds down the trigger and keeps shooting he loses the Acc bonus for the next round and the chance to hit drops to 10... 1 round on average, wasting 12. If he keeps the trigger planted until the mag runs dry, a squip of 4 shots comes out the last turn with a chance of 7 to hit, which he will likely miss. So, aim for a second, 4 hits out of 30 in three seconds. 3 second reload. Seven seconds per magazine.
If he reaimed after each second's fire he dramatically increases the number of hits, every turn of firing becomes the same as the first turn described above. And a tactical reload to ditch the 4 rounds left in the mag after every second burst. Aim, shoot, aim, shoot, reload. Seven seconds per magazine.
I've heard that the active shooting went for 10 minutes and not the entire 20 minute police response time.
Full auto, dump a mag at each victim; 2,571 rounds could have been fired. On average, 342 hits in that time. But this is also one mag per victim, so just 86 people would be injured. Assuming he schlepped 86 pounds of ammo to the school. Which I don't think he did.
Full auto, reaim; 2,571 rounds could have been fired. 514 hits on 172 victims!
Semi-auto with a 30 round mag. Skill 10, Acc 5, Range -3. 12 to hit. Two hits on average per turn of shooting. He liked hitting each target with at least three so lets assume two rounds of shooting per target. Semi-auto gets a follow-up of 1/2 Acc for a second turn at the same target without reaiming (+2 or 9 to hit for the second turn and we'll get generous and assume it hits).
Six shots per victim, 3 hits (aim, fire, fire). Five victims per magazine. 3 second reload. 18 seconds per magazine. 1,000 rounds! 500 hits, 167 victims.
Ten round magazine. Six at one victim, three at the next, reload; Aim at previous victim, fire... 8 seconds per magazine. Five hits and two victims per magazine. 750 shots fired, 375 hits, 150 victims. The timing of the reload is making him aim more often so he's getting more hits per victim.
SEVEN round magazine. Aim, fire, fire, reload; repeat. One victim per magazine, 3 hits per magazine, 6 seconds per magazine. 700 rounds, 300 hits, 100 victims.
Notice that 1/4 the ammunition in the magazine doesn't translate to 1/4 the number of victims or even 1/4 the number of shots fired?
None of this accounts for the time spent moving between victims.
Lets say half his time is spent moving. Notice we can still get double the actual number of victims with just a seven round magazine?
This is why we laugh our fool heads off at the anti-gunners when they propose magazine capacity limits.
Also remember that the rules of a roleplaying game segment the world into neat and discrete turns, the real world is far more fluid and faster.
Mr Glittershine reloaded far more than he needed to, I've read estimates that he left almost as much ammo on the ground as he fired. If he'd been limited to ten round mags he would have reloaded as often, just with a whole magazine instead of dumping a half-expended one each time.
I forgot the other guns!
Bolt-action hunting rifle. Acc is still five. RoF is just 1, so it's Aim, fire, Aim, fire... 15 seconds to reload. 25 seconds per magazine.
12 to hit with every round and on average every single round does.
120 aimed shots in our ten minutes. 120 hits. 40 victims if he does three per victim. Notice that we've still managed to kill more than was done with an AR in real life?