Since there are more random elements for firearms involved in determining success, the standard d4-1 exploding randomizer is always used, with an automatic -1 attribute consideration to compensate for the stacking randomizer.
So, let's say Lu Ordin is firing his shotgun at an opponent 18 yards away, who also has good cover. The opponent's passive defense of 6, plus the cover (8), plus the range penalties (10) requires an 11 total attribute minimum to score a hit. Lu Ordin has a 6 focus, which becomes a 5 + 1d4-1 for the purposes of firing. His first shot (1d4-1 = 2) is a 7, so he misses. He shoots again, but takes his time to aim the shot (1d4-1 = 3, exploding adds 1d4-1 = 2 + 1 for aim) which comes out to 11. Since the shot passed by a gradient of 1, the effect is 1, which for a shotgun means that the opponent staggers and is bleeding. The physical damage is superficial for a hit like this, however, the opponent will have a hard time trying to focus when he's bleeding, and the staggering will leave him vulnerable to the next immediate attack.
One of the reasons why I incorporated the randomizer into all firearms is to account for critical failures. For example, if you roll a 0, you must roll again. If that comes up to a 0, you roll again, and so on. The amount of zeroes is equivalent to the severity. For example, double zeroes is a dud. Triple zeroes is a jam, like a bullet wasn't loaded into the magazine properly. Quadruple zeroes is a weapon failure. If my math is correct, your chances of failure are 1 in 16 for a dud, 1 in 64 for a jam, and 1 in 256 for a complete failure. Probably not true to life, but they sound like reasonable numbers for a game simulation. Of course, not all firearms are equal. Loosely machined weapons like the AK-47 will have a lower rate of failure, as well as simpler weapons, like revolvers.
Sometime later I will post the rules for burst fire.